This module defines the different types of read, write, and cache that can be configured for the storage devices that the Server Administrator storage management service can view.
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 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.
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 Direct I/O and Cache I/O cache policies are:
Cache I/O. Specifies that all reads are buffered in cache memory.
Direct I/O. 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. Direct I/O is also the default setting.