RAID3简介
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RAID level 3, bit-interleaved parity organization, improves on level 2 by
exploiting the fact that disk controllers, unlike memory systems, can detect
whether a sector has been read correctly, so a single parity bit can be used
for error correction, as well as for detection. The idea is as follows: If one of
the sectors gets damaged, the system knows exactly which sector it is, and,
for each bit in the sector, the system can figure out whether it is a 1 or a 0
by computing the parity of the corresponding bits from sectors in the other
disks. If the parity of the remaining bits is equal to the stored parity, the
missing bit is 0; otherwise, it is 1.
RAID level 3 is as good as level 2, but is less expensive in the number of
extra disks (it has only a one-disk overhead), so level 2 is not used in practice.
Figure 10.3d shows the level 3 scheme.
RAID level 3 has two benefits over level 1. It needs only one parity disk
for several regular disks, whereas level 1 needs one mirror disk for every
disk, and thus level 3 reduces the storage overhead. Since reads and writes
of a byte are spread out over multiple disks, with N-way striping of data,
the transfer rate for reading or writing a single block is N times faster than
a RAID level 1 organization using N-way striping. On the other hand, RAID
level 3 supports a lower number of I/O operations per second, since every
exploiting the fact that disk controllers, unlike memory systems, can detect
whether a sector has been read correctly, so a single parity bit can be used
for error correction, as well as for detection. The idea is as follows: If one of
the sectors gets damaged, the system knows exactly which sector it is, and,
for each bit in the sector, the system can figure out whether it is a 1 or a 0
by computing the parity of the corresponding bits from sectors in the other
disks. If the parity of the remaining bits is equal to the stored parity, the
missing bit is 0; otherwise, it is 1.
RAID level 3 is as good as level 2, but is less expensive in the number of
extra disks (it has only a one-disk overhead), so level 2 is not used in practice.
Figure 10.3d shows the level 3 scheme.
RAID level 3 has two benefits over level 1. It needs only one parity disk
for several regular disks, whereas level 1 needs one mirror disk for every
disk, and thus level 3 reduces the storage overhead. Since reads and writes
of a byte are spread out over multiple disks, with N-way striping of data,
the transfer rate for reading or writing a single block is N times faster than
a RAID level 1 organization using N-way striping. On the other hand, RAID
level 3 supports a lower number of I/O operations per second, since every
disk has to participate in every I/O request.
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