Patent Application
20070091500
The method and apparatus relate to magnetic information storage and retrieval and more specifically to a disk drive with improved performance.
An essential component of most modem electronic appliances, including computers, is a disk drive for storing information on magnetic disks or platters. In a hard disk, magnetic recording material is layered onto a high-precision platter (disk) (usually made of glass or aluminum). Typically, the information is stored in concentric tracks usually divided into sectors.
The disk drive includes a reading and recording head (head), a positioning component and a controller. A typical positioning component includes an actuator arm to position the head on the right track on the magnetic platter and a track following system that keeps the head in place.
The actuator arm is controlled by the controller, so that the head, in an initial position, is moved to a target track position. That is referred to as the “seek” phase.
In the seek phase, the actuator arm goes through five phases. In the “speedup” phase the actuator arm is accelerated until it reaches half of the seek distance or a fixed maximum velocity. The actuator arm then enters the “coast” phase where the actuator arm keeps moving at a steady speed. The actuator arm the enters the “slowdown” phase where then the actuator arm is brought to rest very near to the target track. The final phase is the “settle” phase where the controller adjusts the head to access the target track. Once the head is on the right track the controller keeps the head on the desired track until the completion of the data transfer. That is referred to as the “tracking” phase.
During the settling phase the head will oscillate on the desired track for some time as the controller transitions from the seek phase to the tracking phase. Data written while the head is oscillating may not be reliably stored thereby making the data unreadable. To avoid this problem, the controller prevents a write operations for a predetermined time after the transition from the seek mode to the track following mode.
Similarly, a reading operation may be prevented for some time following the transition from seek to track to avoid data from adjacent tracks being inadvertently read. The delay of such read and write operations decreases the performance of the disk drive.
The inefficiency is also seen in erasure-encoded disks during read-before-write and read-modify-write operations. In a typical disk drive, these operations are more time consuming than a write operation by nearly a full rotation of the platters, which, for current disks takes 3 to 12 milliseconds. For many applications this is insignificant, but for others, the amount of buffering required to hide this latency would be onerous.
As an example, consider a computer calculation of a parity value: the” exclusive-or” (a Boolean operator that returns a value of TRUE only if both its operands have different values) of data values A and B. Each write to the platter is best done by reading the previous value, writing the new value, then computing D, the difference (i.e., exclusive-or) between the old and new values. To maintain a parity value, we must then replace the previous parity value with the difference between its old value and D.
In a typical disk drive, the positioning component will move the head into the right general neighborhood of the track currently being read or written, and then make fine adjustments to the head position until it is actually above the correct track. In the case of a read operation, once the data on the surface can be resolved to be the correct sector of the correct track, the result can be returned; the low-level error-correcting codes allow the platter to verify that the bits that have been read all belong to the desired sector. For a write operation, the process takes more time (about 2 milliseconds), because the position of the disk head continues to oscillate slightly. Performing a write operation during these oscillations could be problematic: the data bits written might belong to a sector on a neighboring track, whose value would then be corrupt. While the low-level error-correcting codes on a platter can cope with a small number of such miswrites, a large number would result in the loss of the data from the neighboring sector. As disks become smaller and track densities increase, the fraction of the total seek time attributed to the settle phase increases.
This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.
In an implementation, a disk drive is provided with a second head disposed behind the first head at a distance that enables the second head to write on a sector read by the first head without incurring the cost of a second rotational pass.
In another implementation a method of improving the efficiency of a disk drive is provided that includes the steps of reading a sector with a first head and writing on the sector with a second head before the platter has rotated a full revolution.
The same numbers are used throughout the drawings to reference like features and components. The design, structure, and functional configurations of the disk drive illustrated in the drawings are merely exemplary to illustrate various features and aspects of an improved efficiency disk drive. Any number of different user components, structures, and functions may be implemented in varying configurations to implement embodiments of the improved disk drive.
The disk drive 10 includes a first actuator assembly 17 and a second actuator assembly 19. The first actuator assembly 17 includes an actuator arm 21 that pivots around pivot point 23. A read transducer, or first head 25 is mounted on actuator arm 21. The second actuator assembly 19 similarly includes an actuator arm 27 pivoting around pivot point and supporting a second head 31 that may be a read/write head. Although it may be beneficial for other purposes to allow both heads to read, and possibly both to write, depending on the economics of doing so, and the physics; to the extent that the mass of the assembly is reduced by having single-purpose heads, we may see a corresponding reduction in the settling time for reading and writing. In any case the read head should be placed ahead (in the rotational direction) of the write head by a distance which is roughly equal to the expected difference in settling times for reading and writing.
Disk drive 10 also includes read/write chip 33, which cooperates with heads 25 and 31 to read and or write data from and to the platter 13. In operation, the actuator assemblies 17 and 19 position the heads 25 and 31 on the appropriate track of the platter 13. Although in this illustration the mechanism for positioning the heads 25 and 31 are shown as pivoting actuator arms, any other positioning mechanism can be used, such as a slider mechanism (See
FIG.2 is an illustration of an alternate embodiment of the disk drive 10, wherein the first actuator assembly 17 is disposed adjacent to the second actuator assembly 19. It should be apparent as a result of this disclosure that the structural design and lay-out of the actuator arm assemblies 17 and 19 may take many forms and the applicant does not intend to be limited to any specific layout illustrated in the drawings.
In
α=W*ts;=(3.600 (revs/min)*(1 min/60sec)*(360°/1rev)*(2×1031 3 sec)=43.2°.
For a disk drive with a rotation speed of 7,200 revs/min the optimal angular separation would be 86.4°. For platters spinning significantly faster, the angular separation might exceed 360°, in which case the angle can be reduced by multiples of 360° to find an offset angle.
From the foregoing, it can be seen that the settling time for a write is about a quarter to a half rotation of the platter. By including a second disk actuator arm and head assembly, preferably at the optimal angular separation, or some physically attainable angle of perhaps 240° to 120° degrees close to the optimal angle, ahead of the existing actuator arm and head assembly, one can be sure that at the time by which a sector 37 passes under a stable write head 31 (See
If the two assemblies don't have the same geometry (if, for example, they pivot in opposite directions), one can still bound the range of angular distances established between the heads when addressing the same track. One can establish that the maximum angular distance, and thus the time between a sector passing under the read head and then under the write head, is sufficient to allow for the write head to settle, and that the minimum angular distance provides enough separation most of the time, namely, when the sector has some sufficient fraction of a rotation to perform between the settling of the read head on track, and the sector in question passing under the read head.
Moreover, if both head assemblies are capable of both reading and writing, one can perform the operations with the “wrong” heads, when the rotational position at the time that the two heads settle for reading places the desired sector after the read head, but before the write head.
Because of the slight variability in the settle time depending on the equipment and operations to be performed, the second head may be positioned at an angle greater than, equal to, or slightly less than the optimal angular separation after reducing that separation by multiples of 360°, and still be capable of obtaining efficiency gains.
At least two new operations to the instruction set of the platter are provided. These new operations are: read-before-write, and read-modify-by-XOR-write. Additional operations may be desirable for application beyond simple storage systems, such as performing increments or decrements of certain fields (for performing debit/credit operations in a database), insertions or deletions from a structured value such as a btree node, simple video processing operation, or other computations of sufficient simplicity and regularity and general utility that they can be implemented in the processor of a disk drive.
The illustrative embodiments described in this specification are susceptible to various modifications and alternative constructions. It should be understood, however, that there is no intention to limit the claims to the specific forms disclosed, but on the contrary, the intention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the specification.
