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Chapter 1: Features of the CHALLENGE RAID Storage System
Enhanced Performance: Disk Striping
In disk striping, the SP lays out data records, usually large data records or a
number of small records for the same application, across multiple disks. For
most applications, these disks can be written to or read from simultaneously
and independently. Because multiple sets of read/write heads work on the
same task at once, disk striping can enhance performance.
The amount of information read from or written to each module makes up
the stripe element size (for example, 128 sectors). The stripe size is the
number of data disks in a group multiplied by the stripe element size. For
example, assume a stripe element size of 128 sectors (the default). If the
RAID-5 group has five disks (four data disks and one parity disk), multiply
by 4 the stripe element size of 128 to yield a stripe size of 512 sectors.
Enhanced Performance: Storage System Caching
Caching is available for CHALLENGE RAID storage systems that have two
SPs, each with at least 8 MB of memory, a battery backup unit, and disk
modules in slots A0 through E0. With storage system caching enabled, each
SP temporarily stores requested information in its memory.
Caching can save time in two ways:
•
For a read request, if data is sought after the request is already in the
read cache, the storage system avoids accessing the disk group to
retrieve the data.
•
For a write request, if the information in the write cache is modified by
the request and thus must be written to disk, the SP can keep the
modified data in the cache and write it back to disk at the most
expedient time instead of immediately. Write caching, in particular, can
enhance storage system performance by reducing write time response.
To ensure data integrity, each SP maintains a mirror image of the other SP’s
caches. If one SP fails, the data in its caches is available from the other SP.
You enable storage system caching and specify basic cache parameters, and
enable or disable read and write caches for individual disk units using the
command line interface, as explained in Chapter 7.
