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SANtricity 11.8

Cache settings and performance

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Cache memory is an area of temporary volatile storage on the controller that has a faster access time than the drive media.

With caching, overall I/O performance can be increased as follows:

  • Data requested from the host for a read might already be in the cache from a previous operation, thus eliminating the need for drive access.

  • Write data is written initially to the cache, which frees the application to continue instead of waiting for the data to be written to the drive.

The default cache settings meet the requirements for most environments, but you can change them if you want.

Storage array cache settings

For all volumes in the storage array, you can specify the following values from the System page:

  • Start value for flushing — the percentage of unwritten data in the cache that triggers a cache flush (write to disk). When the cache holds the specified start percentage of unwritten data, a flush is triggered. By default, the controller starts flushing the cache when the cache reaches 80 percent full.

  • Cache block size — the maximum size of each cache block, which is an organizational unit for cache management. The cache block size is by default 8 KiB, but can be set to 4, 8, 16, or 32 KiB. Ideally the cache block size should be set to the predominant I/O size of your applications. File systems or database applications generally use smaller sizes, while a larger size is good for applications requiring large data transfer or sequential I/O.

Volume cache settings

For individual volumes in a storage array, you can specify the following values from the Volumes page (Storage › Volumes):

  • Read caching — The read cache is a buffer that stores data that has been read from the drives. The data for a read operation might already be in the cache from a previous operation, which eliminates the need to access the drives. The data stays in the read cache until it is flushed.

    • Dynamic read cache prefetch — Dynamic cache read prefetch allows the controller to copy additional sequential data blocks into the cache while it is reading data blocks from a drive to the cache. This caching increases the chance that future requests for data can be filled from the cache. Dynamic cache read prefetch is important for multimedia applications that use sequential I/O. The rate and amount of data that is prefetched into cache is self-adjusting based on the rate and request size of the host reads. Random access does not cause data to be prefetched into cache. This feature does not apply when read caching is disabled.

  • Write caching — The write cache is a buffer that stores data from the host that has not yet been written to the drives. The data stays in the write cache until it is written to the drives. Write caching can increase I/O performance.

    Caution

    Possible loss of data — If you enable the Write caching without batteries option and do not have a universal power supply for protection, you could lose data. In addition, you could lose data if you do not have controller batteries and you enable the Write caching without batteries option.

    • Write caching without batteries — The write caching without batteries setting lets write caching continue even when the batteries are missing, failed, discharged completely, or not fully charged. Choosing write caching without batteries is not typically recommended, because data might be lost if power is lost. Typically, write caching is turned off temporarily by the controller until the batteries are charged or a failed battery is replaced.

    • Write caching with mirroring — Write caching with mirroring occurs when the data written to the cache memory of one controller is also written to the cache memory of the other controller. Therefore, if one controller fails, the other can complete all outstanding write operations. Write cache mirroring is available only if write caching is enabled and two controllers are present. Write caching with mirroring is the default setting at volume creation.