HP-UX System Administrator's Guide: Routine Management Tasks

Pros and Cons
You may not write to a RAID 3 array, except in full data stripe logical blocks. This limits
application design flexibility and also the user’s ability to have different arrays run at
different RAID levels on the same system.
RAID 3 is not well suited for multiple process I/O (long or short) and is especially not
suited for any application that requires a high I/O per second rate with any degree of
randomness. On the other hand, RAID 3 will deliver excellent performance for single
process/single stream long sequential I/O requests.
Recommended Uses and Performance Considerations
RAID 3 provides consistently lower I/O performance when compared to standalone
disks except when the I/O size is less than or equal to 64 KB.
RAID 3 architecture should only be chosen in a case where the user is virtually
guaranteed that there will be only a single, long process accessing sequential data. A
video server and a graphics server would be good examples of proper RAID 3
applications. RAID 3 is so limited that it becomes a poor choice in most other cases.
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.
208 Using High Availability Strategies