Dell RAID Notes

Read Policy:
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.


Write Policy:
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.

Cache Policy:
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.

The Cached I/O setting is a Read policy which causes the controller to use the controller's cache in a two step process. Everything is transferred from disk to the cache first, then to the requesting process.

Direct I/O, as it turns out, uses the controller cache also, but more efficiently. It transfers all read request from disk to cache and the host process. Subsequent reads from the same block can then be directly retrieved from the controller's cache

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