Page 264 - DCAP103_Principle of operating system

Unit 7: Secondary Storage Structure

of the corresponding bits from sectors in the other disks. If the parity of the remaining bits is Notes
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. This scheme is shown pictorially in Figure 7.9(d). RAID level 3
has two benefits over level 1. Only one parity disk is needed for several regular disks, unlike
one mirror disk for every disk in level 1, thus reducing 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 as fast as with a RAID-level-1 organization
using N-way striping. On the other hand, RAID level 3 supports a lower number of I/Os per
second, since every disk has to participate in every I/O request. A further performance problem
with RAID 3 (as with all parity-based RAID levels) is the expense of computing and writing the
parity. This overhead results in significantly slower writes, as compared to non-parity RAID
arrays. To moderate this performance penalty, many RAID storage arrays include a hardware
controller with dedicated parity hardware. This offloads the parity computation from the CPU
to the array. The array has a non-volatile RAM (NVRAM) cache as well, to store the blocks
while the parity is computed and to buffer the writes from the controller to the spindles. This
combination can make parity RAID almost as fast as non-parity. In fact, a caching array doing
parity RAID can outperform a non-caching non-parity RAID.
RAID Level 4: RAID level 4, or block-interleaved parity organization, uses block-level striping,
as in RAID 0, and in addition keeps a parity block on a separate disk for corresponding blocks
from N other disks. This scheme is shown pictorially in Figure 7.9(e). If one of the disks fails,
the parity block can be used with the corresponding blocks from the other disks to restore the
blocks of the failed disk.
A block read accesses only one disk, allowing other requests to be processed by the other disks.
Thus, the data-transfer rate for each access is slower, but multiple read accesses can proceed in
parallel, leading to a higher overall I/O rate. The transfer rates for large reads is high, since all
the disks can be read in parallel; large writes also have high transfer rates, since the data and
parity can be written in parallel.
Small independent writes, on the other hand, cannot be performed in parallel. A write of a block
has to access the disk on which the block is stored, as well as the parity disk, since the parity
block has to be updated.
Moreover, both the old value of the parity block and the old value of the block being written
have to be read for the new parity to be computed. This is known as the read-modify-write.
Thus, a single write requires four disk accesses: two to read the two old blocks, and two to
write the two new blocks.
RAID Level 5: RAID level 5, or block-interleaved distributed parity, differs from level 4 by
spreading data and parity among all N + 1 disks, rather than storing data in N disks and parity in
one disk. For each block, one of the disks stores the parity, and the others store data. For example,
with an array of five disks, the parity for the nth block is stored in disk (n mod 5) + 1; the nth
blocks of the other four disks store actual data for that block. This setup is denoted pictorially in
Figure 7.9(f), where the Ps are distributed across all level 1, thus reducing 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 as fast as with a RAID-level-1
organization using N-way striping. On the other hand, RAID level 3 supports a lower number of
I/Os per second, since every disk has to participate in every I/O request. A further performance
problem with RAID 3 (as with all parity-based RAID levels) is the expense of computing and
writing the parity. This overhead results in significantly slower writes, as compared to non-
parity RAID arrays. To moderate this performance penalty, many RAID storage arrays include a
hardware controller with dedicated parity hardware. This offloads the parity computation from
the CPU to the array. The array has a non-volatile RAM (NVRAM) cache as well, to store the
blocks while the parity is computed and to buffer the writes from the controller to the spindles.
This combination can make parity RAID almost as fast as non-parity. In fact, a caching array
doing parity RAID can out perform a non-caching non-parity RAID.

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