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Cluster Planning 23
RAID Levels 2 and 3
RAID 2 and RAID 3 are parallel process array mechanisms, where all drives
in the array operate in unison. Similar to data striping, information to be
written to disk is split into chunks (a fixed amount of data), and each chunk is
written out to the same physical position on separate disks (in parallel). When
a read occurs, simultaneous requests for the data can be sent to each disk.
This architecture requires parity information to be written for each stripe of
data; the difference between RAID 2 and RAID 3 is that RAID 2 can utilize
multiple disk drives for parity, while RAID 3 can use only one. If a drive should
fail, the system can reconstruct the missing data from the parity and
remaining drives.
Performance is very good for large amounts of data but poor for small
requests since every drive is always involved, and there can be no
overlapped or independent operation.
RAID Level 4
RAID 4 addresses some of the disadvantages of RAID 3 by using larger
chunks of data and striping the data across all of the drives except the one
reserved for parity. Using disk striping means that I/O requests need only
reference the drive that the required data is actually on. This means that
simultaneous, as well as independent reads, are possible. Write requests,
however, require a read/modify/update cycle that creates a bottleneck at the
single parity drive. Each stripe must be read, the new data inserted and the
new parity then calculated before writing the stripe back to the disk. The
parity disk is then updated with the new parity, but cannot be used for other
writes until this has completed. This bottleneck means that RAID 4 is not
used as often as RAID 5, which implements the same process but without the
bottleneck. RAID 5 is discussed in the next section.
RAID Level 5
RAID 5, as has been mentioned, is very similar to RAID 4. The difference is
that the parity information is distributed across the same disks used for the
data, thereby eliminating the bottleneck. Parity data is never stored on the
same drive as the chunks that it protects. This means that concurrent read
and write operations can now be performed, and there are performance
increases due to the availability of an extra disk (the disk previously used for
parity). There are other enhancements possible to further increase data
transfer rates, such as caching simultaneous reads from the disks and
transferring that information while reading the next blocks. This can generate
data transfer rates that approach the adapter speed.