User's Manual

omreport storage: Using the Storage Reports 69
• Write Cache — Write policies specify whether the controller sends a write-request completion signal
as soon as the data is in the cache or after it has been written to disk. The write policies are as follows:
– Write-Back Caching — When using write-back caching, the controller sends a write-request
completion signal as soon as the data is in the controller cache but has not yet been written to
disk. Write-back caching may provide improved performance since subsequent read requests can
more quickly retrieve data from the controller cache than they could from the disk. Write-back
caching also entails a data security risk, however, since a system failure could prevent the data from
being written to disk even though the controller has sent a write-request completion signal. In this
case, data may be lost. Other applications may also experience problems when taking actions that
assume the data is available on the disk.
– Write-Through Caching — When using write-through caching, the controller sends a write-
request completion signal only after the data is written to the disk. Write-through caching
provides better data security than write-back caching, since the system assumes the data is
available only after it has been safely written to the disk.
• Cache Policy — Indicates whether read cache, write cache, or both are enabled for a specific disk.
The direct I/O and cache I/O cache policies apply to reads on a specific logical drive. These settings do
not affect the read-ahead policy. The cache I/O and direct I/O cache policies are as follows:
– Cache I/O — Specifies that all reads are buffered in cache memory.
– Direct I/O (default) — Specifies that reads are not buffered in cache memory. When using direct
I/O, data is transferred to the controller cache and the host system simultaneously during a read
request. If a subsequent read request requires data from the same data block, it can be read directly
from the controller cache. The direct I/O setting does not override the cache policy settings.
• Layout — RAID level for the array. See your Array Manager documentation for more information
about RAID levels.
• Size — Amount of storage on the disk in GB.