Managing Systems and Workgroups: A Guide for HP-UX System Administrators

Using High Availability Strategies
Disk Arrays Using RAID Data Protection Strategies
Appendix A952
RAID 5
With this RAID level, both data and encoded data protection information
are spread across all the drives in the array. Level 5 is designed to
provide a high transfer rate (a one-way transmission of data) and a
moderate I/O rate (a two-way transmission of data).
In RAID 5 technology, the hardware reads and writes parity information
to each module in the array. If a module fails, the system processor can
reconstruct all user data from the user data and parity information on
the other disk modules. When a failed disk module is replaced, the
system processor automatically rebuilds the disk array using the
information stored on the remaining modules. The rebuilt disk array
contains an exact replica of the information it would have contained had
the original disk module never failed.
Pros and Cons
RAID 5 requires fewer drives than RAID 1 or RAID 1/0 which is a
combination of RAID 1 and RAID 0. Disk striping is used and parity data
is distributed for optimum performance. In RAID 5, three to sixteen
drives can be configured per group. Five drives to a group are typical.
The data are distributed across multiple drives preventing the I/O
slowdown caused by constant hits on a single drive.
RAID 5 is not quite as robust as RAID 1/0 and can only sustain the loss
of one disk per group.
Recommended Uses and Performance Considerations
RAID 5 is the most versatile RAID level for most applications.
RAID 5 is a good choice where multitasking applications require a large
history database with a high read rate, or a database that uses a normal
or less-than-normal percentage of write operations, where writes are
33% or less of all I/O operations.
RAID 5 provides consistently high performance for large input/output
operations, greater or equal to 64 KB, but poor for smaller I/O sizes.