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

Using High Availability Strategies
Disk Arrays Using RAID Data Protection Strategies
Appendix A 951
Recommended Uses and Performance Considerations
Effective for high performance I/O environments using noncritical data.
Data striping can also prevent “hot spots,” which are caused by constant
hits on a single drive; a specific drive may be accessed so often that it will
slow down I/O traffic, or shorten the life of the drive.
RAID 3
This type of array uses a separate data protection disk to store encoded
data. RAID 3 is designed to provide a high transfer rate.
RAID 3 organizes data by segmenting a user data record into either bit-
or byte-sized chunks and evenly spreading the data across N drives in
parallel. One of the drives acts as a parity drive. In this manner, every
record that is accessed is delivered at the full media rate of the N drives
that comprise the stripe group. The drawback is that every record I/O
stripe accesses every drive in the group.
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.