Electrical computers and digital processing systems: memory – Storage accessing and control – Control technique
Reexamination Certificate
2000-12-29
2003-11-25
Lane, Jack A. (Department: 2186)
Electrical computers and digital processing systems: memory
Storage accessing and control
Control technique
C711S004000, C711S111000, C711S114000
Reexamination Certificate
active
06654862
ABSTRACT:
BACKGROUND
The typical computer disk drive includes a stack of one or more circular platters and at least one mechanical arm with a read/write head for each surface of each of the platters. The read/write heads read data from or write data to circular tracks on the platters as the platters rotate. The term “cylinder” is often used to describe a set of circular tracks, all lying on different disk surfaces at the same radial distance from the centers of the platters.
In accessing a block of addressed data on a particular surface, the mechanical arm positions the corresponding read/write head to the track that stores the data. This movement of the read/write head to the appropriate track is generally called a “seek.” The read/write head accesses the targeted block of data when that block rotates to the position of the read/write head. The amount of time required for completing a read or write operation is dominated by the seek-time of the read/write head, the rotation time of the disk, and the data-transfer time. The seek-time is a function of the physical distance traveled by the mechanical arm in moving to the appropriate track.
“Short-stroking” is a disk-access technique that involves confining the range-of-movement of the mechanical arm to a small portion of the available cylinders in a disk drive, usually those lying only on the outer tracks of the disks in the drive. Short-stroking improves performance by reducing the distance traveled by the mechanical arm during most disk accesses and thus reducing average seek-time.
Many data-storage systems ensure data integrity and availability by “mirroring” the data stored on each disk. Mirroring involves storing a primary copy of data on one disk and a duplicate copy, or “mirror,” on another disk drive.
FIG. 1A
shows a computing system
100
that uses mirroring to ensure the integrity and availability of data. The system
100
includes one or more central processing units (CPU), or processors
105
, that perform certain operations on data in the system. Each processor
105
executes program instructions that are stored, at least temporarily, in one or more volatile or non-volatile memory devices
110
, such as a random-access memory (RAM) or read-only memory (ROM) chipset. Much of the data provided to and produced by the processor
105
during these operations is stored on the disk drives
115
,
120
in one or more disk subsystems
125
. In each subsystem, a hardware controller
130
governs the manner and locations in which data is stored on the disk drives
115
,
120
.
FIG. 1B
shows an implementation in which the mirrored disk drives
115
,
120
are managed by separate hardware controllers
130
,
132
.
FIG. 2
shows a common mirroring technique, in which one disk drive
115
serves as the primary disk and another disk drive
120
serves as the mirror disk. The mirror disk
120
stores a mirror copy of all or some portion of the data stored on the primary disk
115
. In this example, the disk controller makes essentially the entire surface of each disk available for data storage. As a result, the read/write heads must seek over the entire surface of the primary disk
115
for many read and write operations and must seek over the entire surface of the mirror disk
120
for many write operations. Because all read operations are directed to the primary disk
115
, this technique subjects the primary disk to much heavier use than the mirror disk
120
.
FIG. 3
shows a mirroring technique that offers better performance than the technique shown in FIG.
1
. This technique uses the short-stroking effect to limit the distance traveled by the read/write heads and therefore to reduce the seek-time associated with the disks
115
,
120
. The disk controller stores data only on the outer portions
135
,
140
of the disks
115
,
120
, leaving the inner portions
145
,
150
of the disks unused.
As with the technique shown in
FIG. 1
, the disk controller stores the primary copy of all data on one disk
115
and the mirror copy on another disk
120
. Therefore, this technique also subjects one of the disks to much heavier use than the other.
FIG. 4
shows another mirroring technique that relies on the short-stroking effect. With this technique, the disk controller divides the recording tracks on each disk into distinct groups, or extents, and stores primary and mirror copies of data on alternating extents on each disk. In the example shown here, the disk controller stores the primary copy of one set of data on the outer tracks, or outer extent
155
, of one of the disks
115
(“Disk A”) and stores the mirror copy of that data on the outer extent
160
of the other disk
120
(“Disk B”). The disk controller then stores the primary copy of another set of data on the inner tracks, or inner extent
165
, of Disk B and stores the mirror copy of that data on the inner extent
170
of Disk A. This technique more evenly distributes the workloads of the two disks by placing primary copies of data on both disks, thus ensuring that read operations are directed to both disks.
The short-stroking techniques shown in FIG.
3
and
FIG. 4
improve disk-drive performance, as described above, by limiting the distance over which the read/write heads must travel to access data. However, these techniques also leave a large portion of each disk unused, reducing the total storage capacity of the disk drive and thus driving up the cost of disk storage in the system. In all of the techniques of
FIGS. 2-4
, the primary copy and the mirror copy of each block of data start at the same relative positions, or track offsets, on their respective disks.
SUMMARY
A set of storage disks includes a mirrored pair of disks. Each disk in the pair includes a mirror copy of data stored on the other disk in the pair. For each of the disks in the pair, the data forming the mirror copy is physically located between the physical center of the disk and the data that is mirrored on the other disk.
REFERENCES:
DaGiau, Gianna et al., “Delivering System Performance Requirements Using Fewer Drives”, PowerSolutions pp 55-61 Nov. 2002.
Cowart John D.
Lane Jack A.
NCR Corporation
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