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The read policies indicate whether or not the controller should read sequential sectors of the logical drive when seeking data.
- 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.
The 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.
- Write-Back. 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. 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 virtual disk. These settings do not affect the read-ahead policy. The cache policies are as follows:
- 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.