User's Manual

72 omreport storage: Using the Storage Reports
• Read Cache — Read policies indicate whether or not the controller should read sequential sectors of
the logical drive when seeking data. The read policies are as follows:
– Read-Ahead — When using read-ahead policy, the controller reads sequential sectors of the logical
drive when seeking data. Read-ahead policy may improve system performance if the data is
actually written to sequential sectors of the logical drive.
– No-Read-Ahead — Selecting no-read-ahead policy indicates that the controller should not use
read-ahead policy.
– Adaptive Read-Ahead — When using adaptive read-ahead policy, the controller initiates read-
ahead only if the two most recent read requests accessed sequential sectors of the disk. If
subsequent read requests access random sectors of the disk, the controller reverts to no-read-ahead
policy. The controller continues to evaluate whether read requests are accessing sequential sectors
of the disk, and can initiate read-ahead if necessary.
• 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 the cache policy for privileges to read from and write to the disk
cache is enabled or not.
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 on the virtual disk. See your Array Manager documentation for
more information.
• Size — Amount of storage on the virtual disk in GB.