Service manual

Sizing Disk Subsystems
Chapter 4 Hardware Sizing 93
During normal operation, RAID 5 usually offers lower performance than RAID 0,
1+0 and 0+1, as a RAID 5 volume must do four physical I/O operations for every
logical write. The old data and parity are read, two exclusive or operations are
performed, and the new data and parity are written.Read operations do not suffer
the same penalty and thus provide only slightly lower performance than a
standard stripe using an equivalent number of disks. That is, the RAID 5 volume
has effectively one less disk in its stripe because the space is devoted to parity. This
means a RAID 5 volume is generally cheaper than RAID 1+0 and 0+1, because
RAID 5 devotes more of the available disk space to data.
Given the performance issues, RAID 5 is not generally recommended unless the
data is read-only or unless there are very few writes to the volume. Disk arrays
with write caches and fast exclusive or logic engines can mitigate these
performance issues however, making RAID 5 a cheaper, viable alternative to
mirroring for some deployments.
RAID Levels 2, 3, and 4
RAID levels 2 and 3 are good forlarge sequential transfers of data such as video
streaming. Both levels can process only one I/O operation at time, making them
inappropriate for applications demanding random access. RAID 2 is implemented
using Hamming error correction coding (ECC). This means three physical disk
drives are required to store ECC data, making it more expensive than RAID 5, but
less expensive than RAID 1+0 as long as there are more than three disks in the
stripe. RAID 3 uses a bitwise parity method to achieve redundancy. Parity is not
distributed as per RAID 5, but is instead written to a single dedicated disk.
Unlike RAID levels 2 and 3, RAID 4 uses an independent access technique where
multiple disk drives are accessed simultaneously. It uses parity ina manner similar
to RAID 5, except parity is written to a single disk. The parity disk can therefore
become a bottleneck as it is accessedfor every write,effectively serializingmultiple
writes.
Software Volume Managers
Volume managers such as Solaris™ Volume Manager may also be used for
Directory Server disk management. Solaris Volume Manager compares favorably
with other software volume managers for deployment in production
environments.