Power-efficient seek operations in hard disk drive

Details Power-efficient seek operations in hard disk drive Power-efficient seek operations in hard disk drive

2002-03-14

2004-10-26

Sniezek, Andrew L. (Department: 2651)

Dynamic magnetic information storage or retrieval

Automatic control of a recorder mechanism

Controlling the head

C711S111000, C710S039000

Reexamination Certificate

active

06809896

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to the data processing field, and, more particularly, relates to a technique for conserving power in operation of a hard disk drive.
BACKGROUND OF THE INVENTION
In a hard disk drive, data signals are read from and written to a rotating disk by a head which is moved substantially radially with respect to the disk.
FIG. 1
is a simplified block diagram that illustrates conventional circuitry which controls the positioning of a head in a disk drive. Signal conditioning and reading circuitry
10
receives an input signal from arm electronics, which are not shown. A signal output from the signal conditioning and reading circuitry
10
is provided to a head control circuit
12
. The head control circuit
12
processes the signal received from the signal conditioning and reading circuitry
10
and outputs a control signal to an actuator drive circuit
14
. The actuator drive circuit
14
provides a driving signal to a head actuator motor (not shown) which typically is a voice coil motor (VCM).
The head control circuit
12
also receives commands from a host system, which is not shown. The head control circuit
12
includes a processing capability
16
, which may include one or more processors. Also included in the head control circuit
12
are one or more memory units, represented by memory block
18
in FIG.
1
.
The head control circuit
12
responds to commands from the host by moving the head from a current position to a target position. Such operations are referred to as “seeks”, and are intended to bring the head into a stable position at a target track or cylinder.
According to conventional practices, commands from the host may be queued in the head control circuit
12
, and may be selected from the queue for execution in accordance with a process intended to minimize the average access time for execution of commands. According to a known command queue ordering technique, a Shortest Access Time First (SATF) algorithm is employed. According to this algorithm, the command selected for execution from the command queue is that which can be started first. Estimated access times for commands are contained in a seek profile table stored in the memory
18
. The estimated access times indicated by the seek profile table take two factors into account: (a) the time required to seek to and settle at the target track, and (b) the additional time required for the disk to rotate so that the target sector reaches the locus of the head. This additional rotational time is sometimes referred to as “extra latency”.
When a command fails to be executed within the estimated access time indicated by the seek profile table, execution of the command must await an additional rotation of the disk. This phenomenon can degrade the performance of the disk drive, and is referred to as a “miss”. A command may fail to execute within the estimated access time indicated by the seek profile table as a result of factors such as variations in temperature, variations in head position, and external vibration.
The SATF algorithm has been modified according to an invention that is commonly assigned herewith and referred to as DEAT (Delta Expected Access Time). The DEAT SATF algorithm is disclosed in co-pending commonly-assigned patent application Ser. No. 09/638,253, filed Aug. 14, 2000 now U.S. Pat. No. 6,725,327. That patent application is incorporated herein by reference.
In selecting a command for execution from the command queue, the DEAT SATF algorithm considers not only the estimated access time indicated by the seek profile table, but also the probability that the command will be executed within the estimated time. The probability information may be based on actual experience in executing commands, as indicated by a probability table. A simplified example of a probability table provided according to the DEAT SATF algorithm is shown in FIG.
2
. The probability table of
FIG. 2
indicates execution experience for executed commands that have been categorized according to seek distance and estimated extra latency. The seek distance is measured in terms of cylinder groups, and the estimated extra latency is measured in terms of servo ID's. In
FIG. 2
, four columns are shown, corresponding to four extra latency bands, and eleven rows are shown, corresponding to cylinder groups. For each category of commands, the number of misses and the number of “makes” (occasions on which the command executed within the expected time) are recorded.
FIG. 2A
is another example of a probability table used for the DEAT SATF algorithm, this time presenting data gathered from an actual implementation of the DEAT SATF algorithm. In contrast to
FIG. 2
, the table of
FIG. 2A
expresses the command execution experience in terms of percentages corresponding to miss rates. That is, the cell entries in
FIG. 2A
are derived by dividing the number of misses for each category by the total of makes and misses for the category. It will also be observed that the table of
FIG. 2A
has 240 categories, corresponding to 30 cylinder groups by eight extra latency bands. It should also be noted that in the data shown in
FIG. 2A
, for a given category or cell, a miss rate of zero is recorded when the total of makes and misses for that category is less than
10
.
Power consumption may be a consideration in driving a head actuator. In many conventional disk drives, the head is driven to the target cylinder as rapidly as possible, without regard to the estimated extra latency for the command in question. This technique may use more power than is necessary to perform the command without incurring a miss. Power is used by the VCM only while accelerating or decelerating the actuator. To the extent that the head arrives at the target track prior to the target sector in the track rotating to the head locus, power is theoretically wasted.
One technique for reducing the amount of power consumed in driving the head actuator is to reduce (or “clip”) the maximum velocity of the actuator. This reduces the amount of acceleration and deceleration during seek operations, thereby reducing the amount of current drawn by the VCM. However, if the maximum seek velocity is reduced across the board, performance of the disk drive is adversely affected.
It could be contemplated to select a command for execution from the command queue based on considerations of power consumption, e.g., by giving priority to commands which require less power, but this would tend to adversely affect performance.
According to another power saving technique, two seek profile tables are provided, with one of the tables corresponding to a full power maximum seek velocity, and the second table corresponding to a lower power (lower maximum velocity). In operation with the two seek profile tables, the high power seek profile table and an associated probability table are employed in accordance with the DEAT SATF algorithm to select a command for execution from the command queue. Then, the corresponding entry in the lower power seek profile table is consulted to determine whether the selected command can also be expected to execute on time if constrained by the lower maximum velocity. If it is determined that the selected command will likely be executed on time even if executed at lower power, then the lower maximum velocity constraint is applied. Otherwise, the lower maximum velocity constraint is not applied.
This technique provides some savings in power without adversely affecting performance, but requires substantial memory space for storing the additional seek profile table. Furthermore, power savings are less than optimal, since the number of power levels is limited to the number of seek profile tables, and the number of seek profile tables that may be practically provided is limited by memory constraints.
It would be desirable to provide a technique for controlling head actuation that provides a greater reduction in power consumption without adversely affecting performance of the disk drive.
SUMMARY OF THE INVENTION
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