System and method for reducing latency for serially accessible media

FIELD OF THE INVENTION

The present invention relates generally to systems and methods for reading data from, and writing data to, serially accessible storage media such as tape and, more particularly, to systems and methods for storing multiple copies of data on different locations either on the same serially accessible storage media or on a different serially accessible storage media.

BACKGROUND OF THE INVENTION

There are various types of media used for the storage of data. Each media type has particular characteristics that typically dictate the environment/application that it is best suited for. For example, disk media is typically used for real-time data storage when fast access to a particular location on the media is required. Tape media, and in particular magnetic tape, is typically used for off-line data storage of large amounts of data such as a backup or archive copy of data. Disk media is relatively expensive when compared to other types of media such as tape. Tape has the disadvantage of a relatively slow access time, when compared to disk, as the tape is wound about a reel and must be accessed serially by either forwarding or rewinding the tape to the desired location for reading/writing data. It would be desirable to improve the access time for tape media such that the cost benefit of tape could be used in more types of environments/applications that traditionally use disk (with its associated faster access time and lower latency). In addition, with appropriate management of the data, the technique for improving serially accessible media's access time can provide the additional benefit of data redundancy.

As seen in FIGS. 1A–1C, tape media used for the storage of computer data can be packaged in many different forms. FIG. 1A shows a single reel tape cartridge, such as that used with a 9490 tape drive offered by Storage Technology Corporation, headquartered in Louisville, Colo. This tape cartridge 12 has a single supply reel 13 and hub 14 contained therein, with a portion 15 of the tape media 16 wrapped around hub 14 when in a loaded position (i.e. threaded and ready for access by a transducer 20 such as a magnetic read/write head). Another portion of the tape 16 is shown threaded along a tape path defined by a plurality of rollers or capstans 18. Another portion 17 of the tape is shown as being wound about a hub 24 within take-up reel 22. This tape is shown to be in a loaded position within a tape drive—where the tape is adjacent to the transducer 20 for data access by the tape drive. Motors (not shown) are used to drive the hubs 14 and 24 such that the tape can be positioned in a forward or reverse direction such that different linear portions of the tape can be positioned adjacent the transducer 20 for data access (e.g. reading or writing). When in an unloaded position, for example when the cartridge is not loaded in a tape drive, the tape would not extend along the tape path, but rather be exclusively contained within the tape cartridge 12. In a single reel cartridge system, the take-up reel 22 is contained within and is a part of the tape drive.

FIG. 1B shows a dual reel tape cartridge 112, such as that used with a 9840 tape drive also offered by Storage Technology Corporation. This tape cartridge 112 has both a supply reel 113/hub 114 and a take-up reel 122/hub 124 contained therein. Tape 116 is shown threaded along a tape path defined by a plurality of rollers or capstans 118. When loaded in a tape drive, the tape is accessible by a transducer (not shown) via aperture 100. Motors (not shown) are used to drive the hubs 114 and 124 such that the tape can be positioned in a forward or reverse direction such that different linear portions of the tape can be positioned adjacent a transducer for data access (e.g. reading or writing).

FIG. 1C shows a dual reel cassette 212, such as that originally used with a VCR tape recorder/player traditionally used for viewing/recording programming on/off a television or monitor, and now being used for the storage of data using helical recording. Such helical data storage is described in U.S. Pat. No. 5,128,815, which is hereby incorporated by reference as background material. There is a supply reel 213/hub 214 and take-up reel 222/hub 224, with one end of tape 216 attached to the supply reel 213, and the other end of tape 216 attached to the take-up reel 222. Again, a plurality of rollers or capstans 218 are used to define a tape path, which in this instance positions tape outside of cartridge 212 to wrap the tape about a helical transducer 220.

U.S. Pat. No. 6,061,194 describes a technique for writing duplicate data at a fixed azmith angle from the original data on a disk platter, in order to reduce rotational latency when reading the data. This duplicate data is written on the same platter as the original data, and the media is relatively expensive when compared to tape.

It would be advantage to provide a technique for improving access time for serially accessible storage media, and to improve data redundancy in a storage system having such media. Examples of serially accessible media include magnetic tape, optical tape, and charge coupled device (CCD) shift registers.

SUMMARY OF THE INVENTION

A system and method for reducing the access time in a storage system having serially accessible media. One or more duplicate copies of data are maintained at different offset locations on serial media, which in the preferred embodiment is tape (magnetic or optical). When a request is made to read the data, a determination is made as to which copy of the data—either the original data or one of the duplicate copies—will have the shortest access time for accessing the data. Generally, this would be the data copy that will be closest to the data transducer when the tape is positioned for access, such as a tape cartridge being loaded in a tape drive. Once the tape is ready to be accessed, the tape is positioned to access the copy of the data that is in closest linear proximity with the reading transducer. Thus, the copy of the data having the lowest access latency is chosen to satisfy the particular I/O request.

In one embodiment, the duplicate data is located at a different offset location than the original data on the same tape media.

In an alternate embodiment, the duplicate data is located at a different offset location than the original data on a different tape media.

In yet another embodiment, multiple duplicate copies of the original data are maintained at a plurality of differing offset locations, either on the same media, some on the same media and some on different media, or all on different media.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which:

FIG. 1A is a drawing illustrating a single reel tape cartridge shown in a loaded position within a tape drive.

FIG. 1B is a drawing illustrating a dual reel tape cartridge.

FIG. 1C is a drawing illustrating a dual reel cassette.

FIG. 2A is a drawing illustrating a dual reel cassette with a midway load point.

FIG. 2B is a drawing illustrating a dual reel cassette with a majority of tape on the take-up reel.

FIG. 2C is a drawing illustrating a dual reel cassette with a majority of tape on the supply reel.

FIG. 3A is a drawing illustrating a dual reel cartridge with a majority of tape on the take-up reel.

FIG. 3B is a drawing illustrating a dual reel cartridge with a majority of tape on the supply reel.

FIG. 4A is a drawing illustrating a two zone tape configuration.

FIG. 4B is a drawing illustrating a three zone tape configuration.

FIG. 5 (including FIGS. 5A, 5B and 5C) is a drawing illustrating various zones and data offsets for a linear tape.

FIG. 6 is a drawing illustrating various access times for various head access points.

FIG. 7 is a drawing illustrating a data organization/layout used for determining cartridge selection.

FIGS. 8A, 8B and 8C are drawings illustrating sample parameter tables.

FIG. 9 is a graph depicting various calculated relative access times.

FIG. 10 is a drawing illustrating a four zone layout used with biasing of cartridges/cassettes.

FIG. 11 is a drawing illustrating a biasing scheme extended to N cartridges.

FIG. 12 is a drawing illustrating a media library system.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

A dual reel cassette such as that shown in FIG. 1C is shown in various tape wrap stages in FIGS. 2A–2C. FIG. 2A shows a cassette where the tape 216a on supply reel 213 is approximately the same quantity as tape 216b on take-up reel 222. This tape is considered to have a tape load point at the midway point of the tape, the tape load point being the portion of the tape that would be adjacent to or in contact with a transducer in the tape drive when the tape cassette is initially loaded in the tape drive. By maintaining tape in this midway position—as opposed to being fully wound onto either reel—the average time to access a given linear position on the tape (i.e. the access latency) is one-half that of a fully rewound tape.

FIG. 2B shows an example where the majority of the tape 216 is on take-up reel 222 (as shown by 216b), with a relatively smaller portion of tape 216 on supply reel 213 (as shown by 216a). If this were the state of the tape when the cartridge 212 were loaded into a tape drive (not shown), then on average it would be quicker to seek to a given tape data location for tape 216a on supply reel 213 than it would be to seek to a given tape data location for tape 216b on take-up reel 222. In other words, the average access time (i.e. latency) for data residing on the portion of tape on supply reel 213 would be less than the average access time for data residing on the portion of tape on take-up reel 222.

FIG. 2C shows an example where the majority of the tape 216 is on supply reel 213 (as shown by 216a), with a relatively smaller portion of tape 216 on take-up reel 222 (as shown by 216b). If this was the state of the tape when the cartridge 212 were loaded into a tape drive (not shown), then on average it would be quicker to seek to a given tape data location for tape 216b on take-up reel 222 than it would be to seek to a given tape data location for tape 216a on supply reel 213. The average access time for data residing on the portion of tape on take-up reel 222 would be less than the average access time for data residing on the portion of tape on supply reel 213.

FIGS. 3A and 3B show various tape wrap states for a dual cartridge tape cartridge such as that shown in FIG. 1B, with FIG. 3A showing the situation with a majority of tape 116 wrapped on take-up reel 122, and FIG. 3B showing the situation with a majority of tape 116 wrapped on supply reel 113. For the cartridge shown in FIG. 3A, the average access time for data residing on the portion of tape on supply reel 113 would be less than the average access time for data residing on the portion of tape on take-up reel 222. For the cartridge shown in FIG. 3B, the average access time for data residing on the portion of tape on take-up reel 122 would be less than the average access time for data residing on the portion of tape on supply reel 113.

In accordance with the present invention, one or more duplicate copies of data are maintained at different offset locations on serially accessible media. When a request is made to read the data, a determination is made as to which copy of the data will be closest to the data transducer when the tape is positioned for access, such as by being loaded in a tape drive. In other words, a determination is made as to which copy of the data will have the lowest access latency. This is the data copy that is used to satisfy the I/O request. Once the tape is ready to be accessed, the tape is positioned to access this data. In one embodiment, the duplicate data is located at a different offset location than the original data on the same tape media. In an alternate embodiment, the duplicate data is located at a different offset location than the original data on a different tape media. In yet another embodiment, multiple duplicate copies of the original data are maintained at a plurality of differing offset locations, either on the same media, some on the same media and some on different media, or all on different media.

FIG. 4A illustratively shows a tape 116 linearly extended in a stand-alone fashion—i.e. not wrapped on reels. It has a first end 240 (which would normally be attached to a reel when operable in a tape system), a second end 242 (which would normally be attached to either another reel on a two-reel cartridge, or to a leader pin/block in a single reel cartridge), and a middle portion 244. The tape can thus be considered as having two zones 246 and 248, the first zone 246 containing the half of the tape 116 from the first end 240 and extending to the middle portion 244, and the second zone 248 containing the half of the tape 116 from the middle portion 244 and extending to the second end 242.

In the preferred embodiment, if original data is written to a tape location in first zone 246, a duplicate copy of the data is written to a tape location in second zone 248, either on the same tape or on a tape in a different cartridge. This provides an overall reduction in average data access time for subsequent data access, as will now be illustrated with reference to FIGS. 3A and 3B.

Assume that FIG. 3A and FIG. 3B show the identical tape cartridge 112 in differing states. FIG. 3A shows what is defined to be a take-up biased state of cartridge 112, where the take-up reel 122 has a majority of the tape 116 wrapped around it—including all of zone 248 (FIG. 4A) and a portion of zone 246 (FIG. 4A). In the more general case of serially accessible media logically divided into two portions, this state is also known as a first half portion biased state. FIG. 3B shows a supply biased state of cartridge 112, where the supply reel 113 has a majority of the tape 116 wrapped around it —including all of zone 246 (FIG. 4A) and a portion of zone 248 (FIG. 4A). In the more general case of serially accessible media logically divided into two portions, this state is also known as a second half portion biased state. Again, the assumption here is that the original data and the duplicate data are maintained on the same cartridge, but in different zones. Because the tape may be subsequently accessed (for example, by a different computer or process, and regarding different data) after writing the original and duplicate data onto the cartridge, the head access point will likely be different than it was at the completion of writing the original/duplicate data. So, the cartridge may be in an unknown state (e.g. take-up biased state, supply biased state) when a request is received to access (e.g. read or write) original data. Upon receipt of such request, a determination is made as to which copy of the data will have the lowest latency—either the original copy or a duplicate copy—and that data copy is used to satisfy the data access request.

By extension, any number of intermediate portions between the first end and the second end can be defined, thus partitioning the tape into any number of different zones to accommodate the situation where a plurality of duplicate copies of data are to be copied onto the tape. For example, a three zoned (254, 256, 258) tape for accommodating a system that maintains an original copy and two duplicate copies of data is shown in FIG. 4B. As will be later shown, increasing the number of zones decreases the average access latency.

The technique for determining which copy of data has the lowest latency will now be described. Referring to FIG. 5A, a conceptual view of a serially accessible media is shown. This is a representative view only, as the media is generally much longer and has many more data blocks than those shown at 301. Dotted line 300 represents the midpoint of the tape which logical divides the tape into zones 246 and 248. Original data D1 has been written in zone 246 at data block offset 1 (shown at element 301-1) upon receipt of a request to write such data, and duplicate data has been written in zone 248 at data block offset 1 (shown at element 401-1). In this example, D1 and DA have been written at the same block offset from the beginning of each respective zone, which is done for ease of system management. However, there is no fundamental requirement that this in fact be the case in practicing the present invention, as the duplicate data can be at differing offset locations within the other zone. How this might be accomplished will be shown below, where the data block access time calculations are described.

Head access point (HAP) 302 is the tape location that will be adjacent to or in contact with the transducer when the tape is first loaded into a tape drive. For example, FIG. 3B shows head access point 302 of tape 116, which is the location of the tape across from head access aperture 100 when cartridge 112 is first loaded (i.e. has not yet been advanced or rewound) into a tape drive. Returning to FIG. 5A, it can be seen that the time to access the original D1 data (access time #1) is shorter than the time to access the duplicate data DA (access time #2), since the head access point is nearer to D1 than it is to DA. So in this instance, original data D1 is chosen as the copy of data to access in satisfying a data I/O request.

Referring now to FIG. 5B, the head access point 302 in this example is considerably closer to the duplicate data DA, as both the head access point and DA are relatively near the middle of the tape shown as 300. So in this instance, duplicate data DA is chosen as the copy of data to access.

In FIG. 5C, the head access point is to the right of duplicate data DA, so it can be seen that the time to access DA (access time #2) would always be less than the time to access D1 (access time #1), so in this situation duplicate data DA is chosen as the copy of data to access. Although not shown in a figure, but by analogy to FIG. 5C, if the head access point were to the left of original data D1, D1 would be chosen as the copy of data to access in that situation.

As can be seen by the examples shown in FIG. 5, two of the parameters needed to determine the access time for a given data block within a zone are the data block offset within a particular zone, and the zone offset locations within a given tape. For example, original data block D1 is shown at 301-1 to be at offset 1 within zone 246 (the first data block in a given zone being at offset 0 by definition). Similarly, duplicate data block DA is shown at 401-1 to be at offset 1 within zone 248. The relative access times for each data block capable of satisfying a data I/O request (either the original data block, or one of the duplicate data blocks) are calculated, and the smallest access time is then chosen.

For example, in FIG. 5A original data block D1 and duplicate data block DA are both capable of satisfying an I/O request for data block D1. So the relative access times for each of these data blocks is calculated, and the data block having the smallest access time of the two is chosen to be the data block to be used to satisfy the data I/O request. The access time for a given data block within a zone is calculated as follows:

|(head access point)−(zone offset+data offset w/in zone)|

For the example shown in FIG. 5A, with a head access point of 4, the zone and data offset values are shown in the following Table 1:

TABLE 1

D1
DA

Zone Offset
0
13

Data offset w/in zone
1
1

The access time for original data block D1 in FIG. 5A is thus:

D1 access time=|(head access point)−(zone offset+data offset w/in zone)|
=|(4)−(0−1)|
=3

The access time for duplicate data block DA in FIG. 5A is thus:

DA access time=|(head access point)−(zone offset+data offset w/in zone)|
=|(4)−(13+1)|
=16

In this case, original data block D1 would be chosen to satisfy the data I/O request for the scenario in FIG. 5A, since D1<DA.

For the example shown in FIG. 5B, with a head access point of 11, the access time for original data block D1 in FIG. 5B is thus:

D1 access time=|(head access point)−(zone offset+data offset w/in zone)|
=|(11)−(0+1)|
=10

The access time for duplicate data block DA in FIG. 5B is thus:

DA access time=|(head access point)−(zone offset+data offset w/in zone)|
=|(11)−(13+1)|
=3

In this case, duplicate data block DA would be chosen to satisfy the data I/O request for the scenario in FIG. 5B, since DA<D1.

For the example shown in FIG. 5C, with a head access point of 17, the access time for original data block D1 is thus:

D1 access time=|(head access point)−(zone offset+data offset w/in zone)|
=|(17)−(0+1)|
=16

The access time for duplicate data block DA in FIG. 5C is thus:

DA access time=|(head access point)−(zone offset+data offset w/in zone)|
=|(17)−(13+1)|
=3

In this case, duplicate data block DA would be chosen to satisfy the data I/O request for the scenario in FIG. 5C, since DA<D1.

FIG. 6 (including FIGS. 6A–6E) depicts other examples with a representative tape section of thirty (30) blocks divided into two zones. For the examples shown in FIG. 6, the zone and data offset values are shown in the following Table 2:

TABLE 2

D1
DA

Zone Offset
0
15

Data offset w/in zone
2
2

For FIG. 6A, with a HAP of 4, the D1 access is 2 and the DA access is 13, so the D1 data copy would be chosen to satisfy the data I/O request.

For FIG. 6B, with a HAP of 13, the D1 access is 11 and the DA access is 4, so the DA data copy would be chosen to satisfy the data I/O request.

For FIG. 6C, with a HAP of 16, the D1 access is 14 and the DA access is 1, so the DA data copy would be chosen to satisfy the data I/O request.

For FIG. 6D, with a HAP of 20, the D1 access is 18 and the DA access is 3, so the DA data would be chosen to satisfy the data I/O request.

For FIG. 6E, with a HAP of 1, the D1 access is 1 and the DA access is 16, so the D1 data would be chosen to satisfy the data I/O request.

FIG. 7 depicts the preferred embodiment of how to organize and track various parameters of the original and duplicate data, in order to facilitate the above described access time calculations. In the embodiment shown in FIG. 7, a set of information is maintained for each cartridge. Table 500 shows information maintained for cartridge 1, table 600 shows information for cartridge 2, and ending with table 700 with information for cartridge N. Additional tables (not shown) would be used for additional cartridges in the data library. The first field (502, 602, 702) in each respective table contains information that indicates the number (#) of zones defined for this particular cartridge, the size or offset of each zone (which may all be the same size, or may have differing sizes), and the current head access point (HAP) for this cartridge. The next field (504, 604, 704) in each respective table contains directory information as to where the particular original data items D1-D# are physically located on the tape. In the embodiment shown in FIG. 7, this directory information is maintained as zone and data offset information for each data item, identified by data ID, stored on the media. Fields 506, 606 and 706 contain information that identifies the location of the one or more duplicate data items (e.g. DA in FIG. 5) that are associated with the original data item (e.g. D1 in FIG. 5). For example, in field 506 there is specified the physical cartridge, zone and data offset for duplicate data A. This field also accommodates multiple duplicate copies of the data, indicated in the table as extending to duplicate data item Z and its associated physical location information. A set of one or more duplicate data items (DA–DZ) are maintained for each original data item D1–D#. The identification of the physical cartridge(s) in field 506 allows for storing the duplicate copy (ies) either on the same media, some on the same media and some on different media, or all on different media.

Typically, the determination of how many duplicate copies of data are to be maintained in a given tape system environment, and hence the number of zones that are needed to be established, are part of a system initialization or set-up process, and are not modified on a regular or frequent basis. Thus, the zone size/offset parameters tend to be somewhat static in value. However, the head access point value would typically change each time a cartridge completes a drive load/unload sequence. Because of the dynamic changing of this value, it is preferable to maintain the tables shown in FIG. 7 either in system memory, or in an alternate embodiment, in a non-volatile meta-label or meta-tag that is physically affixed to the media cartridge. Maintaining this metadata information in such meta-tag/meta-label reduces overall system memory requirements, as a table for each cartridge in the system would not have to be maintained in system memory, but rather only those tables for the cartridges that are currently being accessed. Such a meta-tag/meta-label system is described in U.S. Pat. Nos. 5,971,281 and 6,081,857, which are hereby incorporated by reference. This metadata that is maintained in a meta-tag/meta-label can then be easily accessed by a robot or robotic arm having an electromagnetic transducer, or alternatively the metadata can be transmitted to a controller via a wireless network.

FIGS. 8A–8C show sample parameter tables 500 representative of each of the sample system states shown in FIGS. 5A–5C, respectively. Referring to FIG. 8A, it can be seen in field 502 that cartridge 1 has two zones, each having a size of 13. The current HAP is 4. Field 504 indicates that original data item having an ID of D1 is stored in zone 1 at data offset 1. Field 506 indicates that duplicate data item DA associated with D1 is stored in the same cartridge 1, in zone 2 at data offset 1. The end-of-field (EOF) 802 indicates that this is the end of the duplicate data list—in other words, there is only one duplicate data item for this particular data item. If there were other duplicate data copies, their locations would listed in succession before the EOF field. In similar fashion, other original data items on this cartridge would be included as entries in field 504, with their associated duplicate copies identified by corresponding entries in field 506. FIGS. 8B and 8C identify the tape state shown in FIGS. 5B and 5C, respectively, with the difference being different HAPs for cartridge 1. As previously discussed, the HAPs typically change each time a data cartridge is accessed by a tape drive.

As can be seen from the organization of the tables shown in FIG. 7, it is very convenient to scale this redundant system to maintain multiple duplicate copies of data—from duplicate blocks DA to DZ in the example shown. This allows the system to be configured to meet the particular needs of a given tape system, which is a key requirement for tape systems, because of the wide gamut of uses of tape—from a high speed access system which emulates a disk system using a combination of front-end cache and a back-end tape system, to a more traditional slow speed access tape system that provides off-line data backup of a disk system during off-hours.

FIG. 9 is a graph depicting the calculated relative access times for a tape having from zero to three duplicate copies of an original data item. For these calculations, the tape was assumed to have 1,800 blocks/track (although tape typically has many more blocks/track), and each data offset for each data copy was assumed to be 2. For a single copy of data (i.e. one with no duplicate copy), the average number of blocks that would have to be traversed to reach the desired data item is 897. Thus, the average access time would be 897 units, with each unit being the amount of time to traverse a given block. For two copies of data (one original and one duplicate), the average access time is 336 units. For three copies of data (one original and two duplicate), the average access time is 199 units. For four copies of data (one original and three duplicate), the average access time is 140 units. Thus, it can be seen that increasing the number of duplicate copies results in a decrease in the average data access time or latency.

Tables 3-1 through 3-18 show the assumptions, data and calculated results used for generating the graph shown in FIG. 9. In particular, the tables show the relative access times for various copies of data (original and duplicate), and the access time for the selected copy having the lowest access time, for various HAP values. It should be noted that not every HAP table entry for a given table is shown, as various data patterns become self-evident without requiring each HAP entry to be shown.

Tables 3-1 through 3-4 show various parameters of a two (2) zone layout with 1800 blocks/track. The D1 Zone Offset is 0, and the DA zone offset is in the middle of the tape at offset 900. The D1 Data offset is at 2 (within zone D1), and the DA data offset is at 2 (within zone DA). The tables shows which copy of data is selected for various head access points (HAP) in this two zone layout. As can be seen in Table 3-1, which shows HAP 0 through HAP 46, the D1 copy of data is selected as it has the smallest access time. Table 3-2 shows HAP 407 through HAP 466, and also shows the transition point (which is circled) where the DA copy of data begins to be selected for HAP greater than 452. Table 3-3 shows that the DA copy of data continues to be selected, and also shows the instance where the HAP 902 coincides with the DA data copy (i.e. where the DA access time is zero, as shown by the table entry highlighted by arrows). Table 3-4 shows HAP 1787 through 1799, where the DA copy of data continues to be selected. It can be seen that the average access time for the selected data is 336.25 units of time, which is less than one half the average access time if only copy D1 where selected (i.e. not taking advantage of selecting the duplicate copy).

Tables 3-5 through 3-9 show various parameters of a three (3) zone layout with 1800 blocks/track. The D1 Zone Offset is 0, the DA zone offset is ⅓ of the way from the beginning of the tape at offset 600, and the DB zone offset is ⅔ of the way from the beginning of the tape at offset 1200. The D1 Data offset is at 2 (within zone D1), the DA data offset is at 2 (within zone DA), and the DB data offset is at 2 (within zone DB). These tables show which copy of data is selected for various head access points (HAP) in this three zone layout. As can be seen in Table 3-5, which shows HAP 0 through HAP 48, the D1 copy of data is selected as it has the smallest access time. Table 3-6 shows HAP 289 through HAP 348, and also shows the transition point (which is circled) where the DA copy of data begins to be selected for HAP greater than 302. Table 3–7 shows that the DA copy of data continues to be selected, and also shows the instance where the HAP 602 coincides with the DA data copy (i.e. where the DA access time is zero, as shown by the table entry highlighted by arrows). Table 3-8 shows HAP 889 through HAP 948, and also shows the transition point (which is circled) where the DB copy of data begins to be selected for HAP greater than 902. Table 3-9 shows that the DB copy of data continues to be selected, and also shows the instance where the HAP 1202 coincides with the DB data copy (i.e. where the DB access time is zero, as shown by the table entry highlighted by arrows). Table 3-10 shows HAP 1789 through 1799, where the DB copy of data continues to be selected. It can be seen that the average access time for the selected data is 199.17 units of time, which is less than one quarter the average access time if only copy D1 where selected (i.e. not taking advantage of selecting the duplicate copy).

Tables 3-11 through 3-18 show various parameters of a four (4) zone layout with 1800 blocks/track. The D1 Zone Offset is 0, the DA zone offset is ¼ of the way from the beginning of the tape at offset 450, the DB zone offset is ½ of the way from the beginning of the tape at offset 900, and the DC zone offset is 1 of the way from the beginning of the tape at offset 1350. The D1 Data offset is at 2 (within zone D1), the DA data offset is at 2 (within zone DA), the DB data offset is at 2 (within zone DB), and the DC data offset is at 2 (within zone DC). These tables show which copy of data is selected for various head access points (HAP) in this four zone layout. As can be seen in Table 3-11, which shows HAP 0 through HAP 48, the D1 copy of data is selected as it has the smallest access time. Table 3-12 shows HAP 169 through HAP 228, and also shows the transition point (which is circled) where the DA copy of data begins to be selected for HAP greater than 227. Table 3-13 shows that the DA copy of data continues to be selected, and also shows the instance where the HAP 452 coincides with the DA data copy (i.e. where the DA access time is zero, as shown by the table entry highlighted by arrows). Table 3-14 shows HAP 649 through HAP 708, and also shows the transition point (which is circled) where the DB copy of data begins to be selected for HAP greater than 677. Table 3-15 shows that the DB copy of data continues to be selected, and also shows the instance where the HAP 902 coincides with the DB data copy (i.e. where the DB access time is zero, as shown by the table entry highlighted by arrows). Table 3-16 shows HAP 1069 through HAP 1128, and also shows the transition point (which is circled) where the DC copy of data begins to be selected for HAP greater than 1127. Table 3-17 shows that the DC copy of data continues to be selected, and also shows the instance where the HAP 1352 coincides with the DC data copy (i.e. where the DC access time is zero, as shown by the table entry highlighted by arrows). Table 3-18 shows HAP 1789 through 1799, where the DC copy of data continues to be selected. It can be seen that the average access time for the selected data is 140.00 units of time, which is less than one sixth the average access time if only copy D1 where selected (i.e. not taking advantage of selecting the duplicate copy).

The previous analysis was based upon random HAPs, where the tape is left in its final position after completion of a tape access operation. It may be desirable to pre-bias to either a supply-reel biased state or a take-up reel biased state after completion of a previous tape access operation, such that the media is maintained in a known state. This would allow for further reductions in data access times. In such a system, the duplicate copy of data is stored on a different cartridge, but not necessarily in a different zone. Instead, the differing cartridges are maintained in different biased states after a previous I/O access, such as writing original and duplicate data. Then, upon receipt of a subsequent I/O request, the cartridge containing the data with the lowest latency is chosen. Again, refer to FIGS. 3A, 3B, 4A and 4B. In this scenario, FIG. 3A shows the cartridge containing the original data in zone 246 (see FIG. 4A), and FIG. 3B shows another cartridge 112 that contains a duplicate copy of the original data also in zone 246 (see FIG. 4A). In this type of system, the two cartridges are maintained to be in differing biased states upon completion of writing the original/duplicate data. The cartridge of FIG. 3A is either fast-forwarded or rewound to be in a take-up reel bias state prior to dismounting the cartridge from the tape drive (after writing the original data). Similarly, the cartridge of FIG. 3B is either fast-forwarded or rewound to be in a supply reel biased state prior to dismounting the cartridge from the tape drive (after writing the duplicate data). The HAP information for each cartridge is then updated to reflect the new HAP resulting from this pre-bias operation. Then, upon a subsequent request to access the original data, either the take-up reel biased cartridge in FIG. 3A or the supply reel biased cartridge in FIG. 3B is chosen depending upon which has the lowest latency, using the calculation techniques previously described.

As one example, assume that the supply reel biased state is defined to be that all tape is on the supply reel—i.e. it is fully rewind after completion of access by a tape drive. This reel will be used to store the duplicate copy of the data item. The take-up reel biased state is defined to be that the tape is positioned to be half on the take-up reel and half on the supply reel (as shown in FIG. 2A). This reel will be used to store the original copy of the data item. For this example, it is assumed that the duplicate copy of data is stored at the same offset location as the original data item, but obviously on a different cartridge. In effect, the duplicate data cartridge is a mirror image of the original data cartridge. Prior to dismounting the cartridges, they are pre-biased as previously described. Upon a subsequent data access request for original data, a determination is made as to the location, or offset, of where this data item resides. The cartridge containing the data with the lowest latency is chosen, using the calculation techniques previously described. FIG. 10 conceptually shows the resultant copy selection, tape 900 can be thought of as being logically divided into four logical zones 906, 908, 910 and 912 of equal size. For data items having a starting location in any of zones 908, 910 and 912, the take-up reel biased cartridge having a head access point (HAP) at the tape midpoint 903 is chosen for data access, as it will have the lowest latency. For data items having a starting location in zone 906, the supply reel biased cartridge having a head access point (HAP) at the tape beginning 902 is chosen for data access, as it will have the lowest latency.

Maintaining cartridges with biased load points can be extended to more than two cartridges. For example, a three cartridge system such as that shown in FIG. 11 could be maintained, with a HAP biased (1) at the beginning of the tape for the first duplicate cartridge, (2) ⅓ of the tape length from the tape beginning for the second duplicate cartridge, and (3) ⅔ of the tape length for the cartridge containing the original data. The cartridge containing duplicate copy #1 would be used to access a data item beginning on the first ⅙ of the tape. The cartridge containing duplicate copy #2 would be used to access a data item beginning between ⅙ and 3/6 of the tape length. The cartridge containing the original copy would be used to access a data item beginning between 3/6 and 6/6 (i.e. the end) of the tape length.

In an alternate embodiment, a race situation is created where at least some of the plurality of media having a copy of the selected data are loaded into respective media drives, and the drive that is first to access the copy of the data provides such data to the requester. In this embodiment, it is preferable to store the duplicate copies of data in different zones on the respective media. A request is received from a requester to read data. A determination is made as to which of a plurality of serially accessible media contain a copy of the requested data. Some or all of media containing a copy of the requested data are loaded into respective media drives. The drives to seek to the copy of the data on their respective media, and the data copy is read. The media drive that is first to access the requested data is used to provide the data to the requestor.

The invention described herein is particularly useful when used in a media library system comprising a plurality of tape drives and media cells. Such a system, as shown at 920 in FIG. 12, advantageously provides numerous cells 926 for holding the increased number of cartridges that may be required to maintain the duplicate copies of data. Such a system also advantageously provides a plurality of media drives 928 that allow concurrent access to a plurality of serially accessible media. The cells 926 and media drives 928 are accessible by a plurality of robots 922 traveling along guides or rails 924. The robots can travel from one row of cells to another, or to a row of media drives, via one or more elevators 930 driven by motors 932. This library system thus allows for data access operations occurring on a first drive and its associated loaded media to overlap either partially or entirely with data access operations of a second drive and its associated loaded media.

Finally, it should be noted that the one or more duplicate copies of data that are maintained to reduce data latency are also available as a redundant copy of data that be can used in lieu of the original data in the event of data loss in the original data, borrowing from techniques used in a traditional data restoration operation.

TABLE 3-1

Duplicate Offsets within Zones - 1800 Blocks/Track

Assumption #1:

1800 blocks/track 2 Zones

D1 Zone Offset
DA Zone Offset

0
900

D1 Data Offset
DA Data Offset

2
2

HAP
D1 Access Time
DA Access Time
Selected Access Time

0
2
902
2

1
1
901
1

2
0
900
0

3
1
899
1

4
2
898
2

5
3
897
3

6
4
896
4

7
5
895
5

8
6
894
6

9
7
893
7

10
8
892
8

11
9
891
9

12
10
890
10

13
11
889
11

14
12
888
12

15
13
887
13

16
14
886
14

17
15
885
15

18
16
884
16

19
17
883
17

20
18
882
18

21
19
881
19

22
20
880
20

23
21
879
21

24
22
878
22

25
23
877
23

26
24
876
24

27
25
875
25

28
26
874
26

29
27
873
27

30
28
872
28

31
29
871
29

32
30
870
30

33
31
869
31

34
32
868
32

35
33
867
33

36
34
866
34

37
35
865
35

38
36
864
36

39
37
863
37

40
38
862
38

41
39
861
39

42
40
860
40

43
41
859
41

44
42
858
42

45
43
857
43

46
44
856
44

TABLE 3-2

D1
DA
Selected

HAP
Access Time
Access Time
Access Time

407
405
495
405

408
406
494
406

409
407
493
407

410
408
492
408

411
409
491
409

412
410
490
410

413
411
489
411

414
412
488
412

415
413
487
413

416
414
486
414

417
415
485
415

418
416
484
416

419
417
483
417

420
418
482
418

421
419
481
419

422
420
480
420

423
421
479
421

424
422
478
422

425
423
477
423

426
424
476
424

427
425
475
425

428
426
474
426

429
427
473
427

430
428
472
428

431
429
471
429

432
430
470
430

433
431
469
431

434
432
468
432

435
433
467
433

436
434
466
434

437
435
465
435

438
436
464
436

439
437
463
437

440
438
462
438

441
439
461
439

442
440
460
440

443
441
459
441

444
442
458
442

445
443
457
443

446
444
456
444

447
445
455
445

448
446
454
446

449
447
453
447

450
448
452
448

451
449
451
449

452
450
450
450

453
451
449
449

454
452
448
448

455
453
447
447

456
454
446
446

457
455
445
445

458
456
444
444

459
457
443
443

460
458
442
442

461
459
441
441

462
460
440
440

463
461
439
439

464
462
438
438

465
463
437
437

466
464
436
436

TABLE 3-3

D1
DA
Selected

HAP
Access Time
Access Time
Access Time

887
885
15
15

888
886
14
14

889
887
13
13

890
888
12
12

891
889
11
11

892
890
10
10

893
891
9
9

894
892
8
8

895
893
7
7

896
894
6
6

897
895
5
5

898
896
4
4

899
897
3
3

900
898
2
2

901
899
1
1

→902
900
0
0←

903
901
1
1

904
902
2
2

905
903
3
3

906
904
4
4

907
905
5
5

908
906
6
6

909
907
7
7

910
908
8
8

911
909
9
9

912
910
10
10

913
911
11
11

914
912
12
12

915
913
13
13

916
914
14
14

917
915
15
15

918
916
16
16

919
917
17
17

920
918
18
18

921
919
19
19

922
920
20
20

923
921
21
21

924
922
22
22

925
923
23
23

926
924
24
24

927
925
25
25

928
926
26
26

929
927
27
27

930
928
28
28

931
929
29
29

932
930
30
30

933
931
31
31

934
932
32
32

935
933
33
33

936
934
34
34

937
935
35
35

938
936
36
36

939
937
37
37

940
938
38
38

941
939
39
39

942
940
40
40

943
941
41
41

944
942
42
42

945
943
43
43

946
944
44
44

TABLE 3-4

D1
DA
Selected

HAP
Access Time
Access Time
Access Time

1787
1785
885
885

1788
1786
886
886

1789
1787
887
887

1790
1788
888
888

1791
1789
889
889

1792
1790
890
890

1793
1791
891
891

1794
1792
892
892

1795
1793
893
893

1796
1794
894
894

1797
1795
895
895

1798
1796
896
896

1799
1797
897
897

Average
897.50
450.00
336.25

Access Time

TABLE 3-5

Assumption #2:

1800 blocks/track 3 Zones

D1 Zone Offset
DA Zone Offset
DB Zone Offset

0
600
1200

D1 Data Offset
DA Data Offset
DB Data Offset

2
2
2

D1
DA
DB
Selected

HAP
Access Time
Access Time
Access Time
Access Time

0
2
602
1202
2

1
1
601
1201
1

2
0
600
1200
0

3
1
599
1199
1

4
2
598
1198
2

5
3
597
1197
3

6
4
596
1196
4

7
5
595
1195
5

8
6
594
1194
6

9
7
593
1193
7

10
8
592
1192
8

11
9
591
1191
9

12
10
590
1190
10

13
11
589
1189
11

14
12
588
1188
12

15
13
587
1187
13

16
14
586
1186
14

17
15
585
1185
15

18
16
584
1184
16

19
17
583
1183
17

20
18
582
1182
18

21
19
581
1181
19

22
20
580
1180
20

23
21
579
1179
21

24
22
578
1178
22

25
23
577
1177
23

26
24
576
1176
24

27
25
575
1175
25

28
26
574
1174
26

29
27
573
1173
27

30
28
572
1172
28

31
29
571
1171
29

32
30
570
1170
30

33
31
569
1169
31

34
32
568
1168
32

35
33
567
1167
33

36
34
566
1166
34

37
35
565
1165
35

38
36
564
1164
36

39
37
563
1163
37

40
38
562
1162
38

41
39
561
1161
39

42
40
560
1160
40

43
41
559
1159
41

44
42
558
1158
42

45
43
557
1157
43

46
44
556
1156
44

47
45
555
1155
45

48
46
554
1154
46

TABLE 3-6

D1
DA
DB
Selected

HAP
Access Time
Access Time
Access Time
Access Time

289
287
313
913
287

290
288
312
912
288

291
289
311
911
289

292
290
310
910
290

293
291
309
909
291

294
292
308
908
292

295
293
307
907
293

296
294
306
906
294

297
295
305
905
295

298
296
304
904
296

299
297
303
903
297

300
298
302
902
298

301
299
301
901
299

302
300
300
900
300

303
301
299
899
299

304
302
298
898
298

305
303
297
897
297

306
304
296
896
296

307
305
295
895
295

308
306
294
894
294

309
307
293
893
293

310
308
292
892
292

311
309
291
891
291

312
310
290
890
290

313
311
289
889
289

314
312
288
888
288

315
313
287
887
287

316
314
286
886
286

317
315
285
885
285

318
316
284
884
284

319
317
283
883
283

320
318
282
882
282

321
319
281
881
281

322
320
280
880
280

323
321
279
879
279

324
322
278
878
278

325
323
277
877
277

326
324
276
876
276

327
325
275
875
275

328
326
274
874
274

329
327
273
873
273

330
328
272
872
272

331
329
271
871
271

332
330
270
870
270

333
331
269
869
269

334
332
268
868
268

335
333
267
867
267

336
334
266
866
266

337
335
265
865
265

338
336
264
864
264

339
337
263
863
263

340
338
262
862
262

341
339
261
861
261

342
340
260
860
260

343
341
259
859
259

344
342
258
858
258

345
343
257
857
257

346
344
256
856
256

347
345
255
855
255

348
346
254
854
254

TABLE 3-7

D1
DA
DB
Selected

HAP
Access Time
Access Time
Access Time
Access Time

589
587
13
613
13

590
588
12
612
12

591
589
11
611
11

592
590
10
610
10

593
591
9
609
9

594
592
8
608
8

595
593
7
607
7

596
594
6
606
6

597
595
5
605
5

598
596
4
604
4

599
597
3
603
3

600
598
2
602
2

601
599
1
601
1

→602
600
0
600
0←

603
601
1
599
1

604
602
2
598
2

605
603
3
597
3

606
604
4
596
4

607
605
5
595
5

608
606
6
594
6

609
607
7
593
7

610
608
8
592
8

611
609
9
591
9

612
610
10
590
10

613
611
11
589
11

614
612
12
588
12

615
613
13
587
13

616
614
14
586
14

617
615
15
585
15

618
616
16
584
16

619
617
17
583
17

620
618
18
582
18

621
619
19
581
19

622
620
20
580
20

623
621
21
579
21

624
622
22
578
22

625
623
23
577
23

626
624
24
576
24

627
625
25
575
25

628
626
26
574
26

629
627
27
573
27

630
628
28
572
28

631
629
29
571
29

632
630
30
570
30

633
631
31
569
31

634
632
32
568
32

635
633
33
567
33

636
634
34
566
34

637
635
35
565
35

638
636
36
564
36

639
637
37
563
37

640
638
38
562
38

641
639
39
561
39

642
640
40
560
40

643
641
41
559
41

644
642
42
558
42

645
643
43
557
43

646
644
44
556
44

647
645
45
555
45

648
646
46
554
46

TABLE 3-8

D1
DA
DB
Selected

HAP
Access Time
Access Time
Access Time
Access Time

889
887
287
313
287

890
888
288
312
288

891
889
289
311
289

892
890
290
310
290

893
891
291
309
291

894
892
292
308
292

895
893
293
307
293

896
894
294
306
294

897
895
295
305
295

898
896
296
304
296

899
897
297
303
297

900
898
298
302
298

901
899
299
301
299

902
900
300
300
300

903
901
301
299
299

904
902
302
298
298

905
903
303
297
297

906
904
304
296
296

907
905
305
295
295

908
906
306
294
294

909
907
307
293
293

910
908
308
292
292

911
909
309
291
291

912
910
310
290
290

913
911
311
289
289

914
912
312
288
288

915
913
313
287
287

916
914
314
286
286

917
915
315
285
285

918
916
316
284
284

919
917
317
283
283

920
918
318
282
282

921
919
319
281
281

922
920
320
280
280

923
921
321
279
279

924
922
322
278
278

925
923
323
277
277

926
924
324
276
276

927
925
325
275
275

928
926
326
274
274

929
927
327
273
273

930
928
328
272
272

931
929
329
271
271

932
930
330
270
270

933
931
331
269
269

934
932
332
268
268

935
933
333
267
267

936
934
334
266
266

937
935
335
265
265

938
936
336
264
264

939
937
337
263
263

940
938
338
262
262

941
939
339
261
261

942
940
340
260
260

943
941
341
259
259

944
942
342
258
258

945
943
343
257
257

946
944
344
256
256

947
945
345
255
255

948
946
346
254
254

TABLE 3-9

D1
DA
DB
Selected

HAP
Access Time
Access Time
Access Time
Access Time

1189
1187
587
13
13

1190
1188
588
12
12

1191
1189
588
11
11

1192
1190
590
10
10

1193
1191
591
9
9

1194
1192
592
8
8

1195
1193
593
7
7

1196
1194
594
6
6

1197
1195
595
5
5

1198
1196
596
4
4

1199
1197
597
3
3

1200
1198
598
2
2

1201
1199
599
1
1

→1202
1200
600
0
0←

1203
1201
601
1
1

1204
1202
602
2
2

1205
1203
603
3
3

1206
1204
604
4
4

1207
1205
605
5
5

1208
1206
606
6
6

1209
1207
607
7
7

1210
1208
608
8
8

1211
1209
609
9
9

1212
1210
610
10
10

1213
1211
611
11
11

1214
1212
612
12
12

1215
1213
613
13
13

1216
1214
614
14
14

1217
1215
615
15
15

1218
1216
616
16
16

1219
1217
617
17
17

1220
1218
618
18
18

1221
1219
619
19
19

1222
1220
620
20
20

1223
1221
621
21
21

1224
1222
622
22
22

1225
1223
623
23
23

1226
1224
624
24
24

1227
1225
625
25
25

1228
1226
626
26
26

1229
1227
627
27
27

1230
1228
628
28
28

1231
1229
629
29
29

1232
1230
630
30
30

1233
1231
631
31
31

1234
1232
632
32
32

1235
1233
633
33
33

1236
1234
634
34
34

1237
1235
635
35
35

1238
1238
636
36
36

1239
1237
637
37
37

1240
1238
638
38
38

1241
1239
639
39
39

1242
1240
640
40
40

1243
1241
641
41
41

1244
1242
642
42
42

1245
1243
643
43
43

1246
1244
644
44
44

1247
1245
645
45
45

1248
1246
646
46
46

TABLE 3-10

D1
DA
DB
Selected

HAP
Access Time
Access Time
Access Time
Access Time

1789
1787
1187
587
587

1790
1788
1188
588
588

1791
1789
1189
589
589

1792
1790
1190
590
590

1793
1791
1191
591
591

1794
1792
1192
592
592

1795
1793
1193
593
593

1796
1794
1194
594
594

1797
1795
1195
595
595

1798
1796
1196
596
596

1799
1797
1197
597
597

Average
897.50
499.17
500.84
199.17

Access

Time

TABLE 3-11

Assumption #3:

1800 blocks/track 4 Zones

D1 Zone Offset
DA Zone Offset
DB Zone Offset
DC Zone Offset

0
450
900
1350

D1 Data Offset
DA Data Offset
DB Data Offset
DC Data Offset

2
2
2
2

D1
DA
DB
DC

Access
Access
Access
Access
Selected

HAP
Time
Time
Time
Time
Access Time

0
2
452
902
1352
2

1
1
451
901
1351
1

2
0
450
900
1350
0

3
1
449
899
1349
1

4
2
448
898
1348
2

5
3
447
897
1347
3

6
4
446
896
1346
4

7
5
445
895
1345
5

8
6
444
894
1344
6

9
7
443
893
1343
7

10
8
442
892
1342
8

11
9
441
891
1341
9

12
10
440
890
1340
10

13
11
439
889
1339
11

14
12
438
888
1338
12

15
13
437
887
1337
13

16
14
436
886
1336
14

17
15
435
885
1335
15

18
16
434
884
1334
16

19
17
433
883
1333
17

20
18
432
882
1332
18

21
19
431
881
1331
19

22
20
430
880
1330
20

23
21
429
879
1329
21

24
22
428
878
1328
22

25
23
427
877
1327
23

26
24
426
876
1326
24

27
25
425
875
1325
25

28
26
424
874
1324
26

29
27
423
873
1323
27

30
28
422
872
1322
28

31
29
421
871
1321
29

32
30
420
870
1320
30

33
31
419
869
1319
31

34
32
418
868
1318
32

35
33
417
867
1317
33

36
34
416
866
1316
34

37
35
415
865
1315
35

38
36
414
864
1314
36

39
37
413
863
1313
37

40
38
412
862
1312
38

41
39
411
861
1311
39

42
40
410
860
1310
40

43
41
409
859
1309
41

44
42
408
858
1308
42

45
43
407
857
1307
43

46
44
406
856
1306
44

47
45
405
855
1305
45

48
46
404
854
1304
46

TABLE 3-12

D1

Access
DA
DB
DC
Selected

HAP
Time
Access Time
Access Time
Access Time
Access Time

169
167
283
733
1183
167

170
168
282
732
1182
168

171
169
281
731
1181
169

172
170
280
730
1180
170

173
171
279
729
1179
171

174
172
278
728
1178
172

175
173
277
727
1177
173

176
174
276
726
1176
174

177
175
275
725
1175
175

178
176
274
724
1174
176

179
177
273
723
1173
177

180
178
272
722
1172
178

181
179
271
721
1171
179

182
180
270
720
1170
180

183
181
269
719
1169
181

184
182
268
718
1168
182

185
183
267
717
1167
183

186
184
266
716
1166
184

187
185
265
715
1165
185

188
186
264
714
1164
186

189
187
263
713
1163
187

190
188
262
712
1162
188

191
189
261
711
1161
189

192
190
260
710
1160
190

193
191
259
709
1159
191

194
192
258
708
1158
192

195
193
257
707
1157
193

196
194
256
706
1156
194

197
195
255
705
1155
195

198
196
254
704
1154
196

199
197
253
703
1153
197

200
198
252
702
1152
198

201
199
251
701
1151
199

202
200
250
700
1150
200

203
201
249
699
1149
201

204
202
248
698
1148
202

205
203
247
697
1147
203

206
204
246
696
1146
204

207
205
245
695
1145
205

208
206
244
694
1144
206

209
207
243
693
1143
207

210
208
242
692
1142
208

211
209
241
691
1141
209

212
210
240
690
1140
210

213
211
239
689
1139
211

214
212
238
688
1138
212

215
213
237
687
1137
213

216
214
236
686
1136
214

217
215
235
685
1135
215

218
216
234
684
1134
216

219
217
233
683
1133
217

220
218
232
682
1132
218

221
219
231
681
1131
219

222
220
230
680
1130
220

223
221
229
679
1129
221

224
222
228
678
1128
222

225
223
227
677
1127
223

226
224
226
676
1126
224

227
225
225
675
1125
225

228
226
224
674
1124
224

TABLE 3-13

D1
DA

Access
Access
DB
DC
Selected

HAP
Time
Time
Access Time
Access Time
Access Time

409
407
43
493
943
43

410
408
42
492
942
42

411
409
41
491
941
41

412
410
40
490
940
40

413
411
39
489
939
39

414
412
38
488
938
38

415
413
37
487
937
37

416
414
36
486
936
36

417
415
35
485
935
35

418
416
34
484
934
34

419
417
33
483
933
33

420
418
32
482
932
32

421
419
31
481
931
31

422
420
30
480
930
30

423
421
29
479
929
29

424
422
28
478
928
28

425
423
27
477
927
27

426
424
26
476
926
26

427
425
25
475
925
25

428
426
24
474
924
24

429
427
23
473
923
23

430
428
22
472
922
22

431
429
21
471
921
21

432
430
20
470
920
20

433
431
19
469
919
19

434
432
18
468
918
18

435
433
17
467
917
17

436
434
16
466
916
16

437
435
15
465
915
15

438
436
14
464
914
14

439
437
13
463
913
13

440
438
12
462
912
12

441
439
11
461
911
11

442
440
10
460
910
10

443
441
9
459
909
9

444
442
8
458
908
8

445
443
7
457
907
7

446
444
6
456
906
6

447
445
5
455
905
5

448
446
4
454
904
4

449
447
3
453
903
3

450
448
2
452
902
2

451
449
1
451
901
1

→452
450
0
450
900
0←

453
451
1
449
899
1

454
452
2
448
898
2

455
453
3
447
897
3

456
454
4
446
896
4

457
455
5
445
895
5

458
456
6
444
894
6

459
457
7
443
893
7

460
458
8
442
892
8

461
459
9
441
891
9

462
460
10
440
890
10

463
461
11
439
889
11

464
462
12
438
888
12

465
463
13
437
887
13

466
464
14
436
886
14

467
465
15
435
885
15

468
466
16
434
884
16

TABLE 3-14

D1

Access
DA
DB
DC
Selected

HAP
Time
Access Time
Access Time
Access Time
Access Time

649
647
197
253
703
197

650
648
198
252
702
198

651
649
199
251
701
199

652
650
200
250
700
200

653
651
201
249
699
201

654
652
202
248
698
202

655
653
203
247
697
203

656
654
204
246
696
204

657
655
205
245
695
205

658
656
206
244
694
206

659
657
207
243
693
207

660
658
208
242
692
208

661
659
209
241
691
209

662
660
210
240
690
210

663
661
211
239
689
211

664
662
212
238
688
212

665
663
213
237
687
213

666
664
214
236
686
214

667
665
215
235
685
215

668
666
216
234
684
216

669
667
217
233
683
217

670
668
218
232
682
218

671
669
219
231
681
219

672
670
220
230
680
220

673
671
221
229
679
221

674
672
222
228
678
222

675
673
223
227
677
223

676
674
224
226
676
224

677
675
225
225
675
225

678
676
226
224
674
224

679
677
227
223
673
223

680
678
228
222
672
222

681
679
229
221
671
221

682
680
230
220
670
220

683
681
231
219
669
219

684
682
232
218
668
218

685
683
233
217
667
217

686
684
234
216
666
216

687
685
235
215
665
215

688
686
236
214
664
214

689
687
237
213
663
213

690
688
238
212
662
212

691
689
239
211
661
211

692
690
240
210
660
210

693
691
241
209
659
209

694
692
242
208
658
208

695
693
243
207
657
207

696
694
244
206
656
206

697
695
245
205
655
205

698
696
246
204
654
204

699
697
247
203
653
203

700
698
248
202
652
202

701
699
249
201
651
201

702
700
250
200
650
200

703
701
251
199
649
199

704
702
252
198
648
198

705
703
253
197
647
197

706
704
254
196
646
196

707
705
255
195
645
195

708
706
256
194
644
194

TABLE 3-15

D1
DA

Access
Access
DB
DC
Selected

HAP
Time
Time
Access Time
Access Time
Access Time

889
887
437
13
463
13

890
888
438
12
462
12

891
889
439
11
461
11

892
890
440
10
460
10

893
891
441
9
459
9

894
892
442
8
458
8

895
893
443
7
457
7

896
894
444
6
456
6

897
895
445
5
455
5

898
896
446
4
454
4

899
897
447
3
453
3

900
898
448
2
452
2

901
899
449
1
451
1

→902
900
450
0
450
0←

903
901
451
1
449
1

904
902
452
2
448
2

905
903
453
3
447
3

906
904
454
4
446
4

907
905
455
5
445
5

908
906
456
6
444
6

909
907
457
7
443
7

910
908
458
8
442
8

911
909
459
9
441
9

912
910
460
10
440
10

913
911
461
11
439
11

914
912
462
12
438
12

915
913
463
13
437
13

916
914
464
14
436
14

917
915
465
15
435
15

918
916
466
16
434
16

919
917
467
17
433
17

920
918
468
18
432
18

921
919
469
19
431
19

922
920
470
20
430
20

923
921
471
21
429
21

924
922
472
22
428
22

925
923
473
23
427
23

926
924
474
24
426
24

927
925
475
25
425
25

928
926
476
26
424
26

929
927
477
27
423
27

930
928
478
28
422
28

931
929
479
29
421
29

932
930
480
30
420
30

933
931
481
31
419
31

934
932
482
32
418
32

935
933
483
33
417
33

936
934
484
34
416
34

937
935
485
35
415
35

938
936
486
36
414
36

939
937
487
37
413
37

940
938
488
38
412
38

941
939
489
39
411
39

942
940
490
40
410
40

943
941
491
41
409
41

944
942
492
42
408
42

945
943
493
43
407
43

946
944
494
44
406
44

947
945
495
45
405
45

948
946
496
48
404
46

TABLE 3-16

DB

D1
DA
Access
DC
Selected

HAP
Access Time
Access Time
Time
Access Time
Access Time

1069
1067
617
167
283
167

1070
1068
618
168
282
168

1071
1069
619
169
281
169

1072
1070
620
170
280
170

1073
1071
621
171
279
171

1074
1072
622
172
278
172

1075
1073
623
173
277
173

1076
1074
624
174
276
174

1077
1075
625
175
275
175

1078
1076
626
176
274
176

1079
1077
627
177
273
177

1080
1078
628
178
272
178

1081
1079
629
179
271
179

1082
1080
630
180
270
180

1083
1081
631
181
269
181

1084
1082
632
182
268
182

1085
1083
633
183
267
183

1086
1084
634
184
266
184

1087
1085
635
185
265
185

1088
1086
636
186
264
186

1089
1087
637
187
263
187

1090
1088
638
188
262
188

1091
1089
639
189
261
189

1092
1090
640
190
260
190

1093
1091
641
191
259
191

1094
1092
642
192
258
192

1095
1093
643
193
257
193

1096
1094
644
194
256
194

1097
1095
645
195
255
195

1098
1096
646
196
254
196

1099
1097
647
197
253
197

1100
1098
648
198
252
198

1101
1099
649
199
251
199

1102
1100
650
200
250
200

1103
1101
651
201
249
201

1104
1102
652
202
248
202

1105
1103
653
203
247
203

1106
1104
654
204
246
204

1107
1105
655
205
245
205

1108
1106
656
206
244
206

1109
1107
657
207
243
207

1110
1108
658
208
242
208

1111
1109
659
209
241
209

1112
1110
660
210
240
210

1113
1111
661
211
239
211

1114
1112
662
212
238
212

1115
1113
663
213
237
213

1116
1114
664
214
236
214

1117
1115
665
215
235
215

1118
1116
666
216
234
216

1119
1117
667
217
233
217

1120
1118
668
218
232
218

1121
1119
669
219
231
219

1122
1120
670
220
230
220

1123
1121
671
221
229
221

1124
1122
672
222
228
222

1125
1123
673
223
227
223

1126
1124
674
224
226
224

1127
1125
675
225
225
225

1128
1126
676
226
224
224

TABLE 3-17

DA

DC

D1
Access
DB
Access
Selected

HAP
Access Time
Time
Access Time
Time
Access Time

1309
1307
857
407
43
43

1310
1308
858
408
42
42

1311
1309
859
409
41
41

1312
1310
860
410
40
40

1313
1311
861
411
39
39

1314
1312
862
412
38
38

1315
1313
863
413
37
37

1316
1314
864
414
36
36

1317
1315
865
415
35
35

1318
1316
866
416
34
34

1319
1317
867
417
33
33

1320
1318
868
418
32
32

1321
1319
869
419
31
31

1322
1320
870
420
30
30

1323
1321
871
421
29
29

1324
1322
872
422
28
28

1325
1323
873
423
27
27

1326
1324
874
424
26
26

1327
1325
875
425
25
25

1328
1326
876
426
24
24

1329
1327
877
427
23
23

1330
1328
878
428
22
22

1331
1329
879
429
21
21

1332
1330
880
430
20
20

1333
1331
881
431
19
19

1334
1332
882
432
18
18

1335
1333
883
433
17
17

1336
1334
884
434
16
16

1337
1335
885
435
15
15

1338
1336
886
436
14
14

1339
1337
887
437
13
13

1340
1338
888
438
12
12

1341
1339
889
439
11
11

1342
1340
890
440
10
10

1343
1341
891
441
9
9

1344
1342
892
442
8
8

1345
1343
893
443
7
7

1346
1344
894
444
6
6

1347
1345
895
445
5
5

1348
1346
896
446
4
4

1349
1347
897
447
3
3

1350
1348
898
448
2
2

1351
1349
899
449
1
1

→1352
1350
900
450
0
0←

1353
1351
901
451
1
1

1354
1352
902
452
2
2

1355
1353
903
453
3
3

1356
1354
904
454
4
4

1357
1355
905
455
5
5

1358
1356
906
456
6
6

1359
1357
907
457
7
7

1360
1358
908
458
8
8

1361
1359
909
459
9
9

1362
1360
910
460
10
10

1363
1361
911
461
11
11

1364
1362
912
462
12
12

1365
1363
913
463
13
13

1366
1364
914
464
14
14

1367
1365
915
465
15
15

1368
1366
916
466
16
16

TABLE 3-18

DA

DC

D1
Access
DB
Access
Selected

HAP
Access Time
Time
Access Time
Time
Access Time

1789
1787
1337
887
437
437

1790
1788
1338
888
438
438

1791
1789
1339
889
439
439

1792
1790
1340
890
440
440

1793
1791
1341
891
441
441

1794
1792
1342
892
442
442

1795
1793
1343
893
443
443

1796
1794
1344
894
444
444

1797
1795
1345
895
445
445

1798
1796
1346
896
446
446

1799
1797
1347
897
447
447

Average
897.50
561.25
450.00
563.75
140.00

Access

Time

Number	Name	Date	Kind
4796110	Glass et al.	Jan 1989	A
5463758	Ottesen	Oct 1995	A
5623471	Prigge	Apr 1997	A
5883864	Saliba	Mar 1999	A
6061194	Bailey	May 2000	A
6662281	Ballard et al.	Dec 2003	B2

System and method for reducing latency for serially accessible media

Information

Patent Number

Date Filed

Date Issued

Inventors

Original Assignees

Examiners

Agents

CPC

US Classifications

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US

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Abstract

Description

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US Referenced Citations (6)