Disk array device

Details Disk array device Disk array device

1998-04-23

2002-06-18

Beausoleil, Robert (Department: 2184)

Error detection/correction and fault detection/recovery

Data processing system error or fault handling

Reliability and availability

C714S049000

Reexamination Certificate

active

06408400

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a disk array device for executing data I/O processing by concurrently accessing a plurality of disk devices, and more specifically, a disk array device for maintaining consistency of data by executing, when write processing is interrupted due to, for instance, power failure, recovery processing of the write processing using the data stored therein.
BACKGROUND OF THE INVENTION
A disk device having nonvolatile memory, a large capacity, a capability for high speed data transfer, and other features such as a magnetic disk or an optical disk device has been widely used as an external storage device for a computer system. Demands for a disk device include high speed data transfer, high reliability, a large capacity, and a low price. A disk array device satisfies the requirements described above. The disk array device comprises a plurality of compact disk devices for distributing and recording data therein, and for enabling concurrent access to the data.
With the disk array device, by concurrently executing data transfer to a plurality of disk devices, data transfer can be executed at a higher rate than the data transfer rate of a single disk device. Further, by recording, in addition to data, redundant information such as parity data, it becomes possible to detect and correct a data error caused by, for instance, a failure of a disk device. Also, a reliability as high as that obtained by duplicating contents of a disk device may be achieved with a lower cost.
It is generally recognized that a disk array device is a new recording medium simultaneously satisfying the three requirements for low price, high speed, and high reliability. The requirement that is most important and most difficult to maintain is high reliability. A single disk constituting a disk array is low in cost, and does not require high reliability. Accordingly, to realize a disk array device, high reliability must be maintained.
David A. Patterson and others at the University of California at Berkeley published reports in which disk array devices that provide redundancy of data by distributing a large volume of data to a number of disks at a high speed are classified from levels 1 to 5 (ACM SIGMOD Conference, Chicago, Ill., Jun. 1-3, 1988, pp.109-116).
The classification of disk array devices proposed by Patterson et al. is abbreviated as RAID (Redundant Array of Independent Disks). Next, brief descriptions are provided for RAID
0
to
5
.
FIG. 32
shows a RAID
0
disk array device. In a RAID
0
disk array device, as shown by data A to I, a disk array control unit
10
distributes data to disk devices
32
-
1
to
32
-
3
according to an I/O request from a host computer
18
, and data reliability for disk error is not insured.
A RAID
1
disk array device has, as shown in
FIG. 33
, a mirror disk device
32
-
2
in which copies A′ to C′ of data A to C stored in the disk device
32
-
1
are stored. For RAID
1
, use efficiency of the disk device is low, but data reliability is insured and can be realized with simple controls, resulting in this type of disk array device being widely used.
A RAID
2
disk array device stripes (divides) data in units of a bit or a byte, and concurrently executes data write or data read to and from each disk device. The striped data is recorded in the same physical sectors in all the disk devices. Hamming code generated from data is used as error correction code. The RAID
2
disk array device has, in addition to disk devices for data storage, a disk device for recording the Hamming code therein, and identifies a faulty disk from the Hamming code to restore data. By having data redundancy based on the Hamming code, data reliability can be insured, even if a disk device fails, but the use efficiency of disk devices is rather low, so that this type of disk array device has not been put into practical use.
A RAID
3
disk array device has the configuration as shown in FIG.
34
. As shown in
FIG. 35
, for instance, data a, b, and c are divided by units of a bit or a sector to data a
1
to a
3
, b
1
to b
3
, and c
1
to c
3
. Parity p
1
is computed from the data a
1
to a
3
, parity p
2
is computed from the data b
1
to b
3
, and parity p
3
is computed from data c
1
to c
3
. The disk devices
32
-
1
to
32
-
4
shown in
FIG. 34
are concurrently accessed to write the data therein.
In a case of RAID
3
, redundancy of data is maintained with parity. Further, a time required for data write can be reduced by concurrently processing the divided data. However, a concurrent seek operation is required for all the disk devices
32
-
1
to
32
-
4
for each access for data write or data read. This scheme is effective when a large volume of data is continuously treated. However, in the case of, for instance, transaction processing for accessing a small volume of data at random, the capability for high-speed data transfer cannot be effectively used, and efficiency is lowered.
A RAID
4
disk array device divides one piece of data by sector and then writes the divided data in the same disk device. For instance, as shown in
FIG. 36
, in the disk device
32
-
1
, data a is divided into sector data a
1
to a
4
and the divided data is written therein. The parity is stored in a disk device
32
-
4
unequivocally decided. Herein parity p
1
is computed from data a
1
, b
1
, and c
1
, parity p
2
from data a
2
, b
2
, and c
2
, parity p
3
from data a
3
, b
3
, and c
3
, and parity p
4
from data a
4
, b
4
, and c
4
.
Data can concurrently be read from the disk devices
32
-
1
to
32
-
3
. When reading data a, sector data a
1
to a
4
are successively read out and synthesized by accessing sectors 0 to 3 of the disk device
32
-
1
. When writing data, data prior to write processing and the parity are read and then new parity is computed to write the data. Thus, the disk device
32
-
1
is accessed a total of 4 times for one write operation.
For instance, when sector data a
1
in the disk device
32
-
1
is updated (rewritten), in addition to data write updating, operations are required for reading old data (a
1
old) at an updated position and old parity (p
1
old) of the corresponding disk device
32
-
4
, computing new parity (p
1
new) consistent with the new data (a
1
new), and then writing the data.
Also, when writing data, the disk device
32
-
4
for parity is always accessed so that data cannot be simultaneously written in a plurality of disk devices. For instance, even if it is tried to simultaneously write data a
1
in the disk device
32
-
1
and data b
2
in the disk device
32
-
2
, it is required to read the parities p
1
and p
2
from the same disk device
32
-
4
and then write the data after computing new parities. Thus the data cannot be simultaneously written in the disk devices.
RAID
4
is defined as described above, but this type of disk array device provides few merits, so there is no actual movement for introduction of this type of disk array device into practical use.
In a RAID
5
disk array device, a disk device is not dedicated for parity, so operations for data read and data write can be concurrently executed. As shown in
FIG. 37
, parities for sectors are written in different disk devices, respectively. Herein parity pl is computed from data a
1
, b
1
, and c
1
, parity p
2
from data a
2
, b
2
, and d
2
, parity p
3
from data a
3
, c
3
, and d
3
, and parity p
4
from data b
4
, c
4
, and d
4
.
As for concurrent operations for data read and data write, for instance, data a
1
for sector 0 of the disk device
32
-
1
and data b
2
for sector 1 of the disk device
32
-
2
are placed in the disk devices
324
and
32
-
3
having parity p
1
and parity p
2
different from each other respectively, so that the operations for reading data and writing data can be concurrently executed. It should be noted that the overhead required for four accesses is the same as that for RAID
4
.
As described above, for RAID
5
, operations for data read and data write can be concurrently executed by accessing a pluralit

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