HP Surestore Disk Array FC60 Disk Array FC60 User's Guide (A5277A) - Page 132
Cache flush threshold default 80
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Setting: Cache flush threshold (default 80%) Function: Sets the level at which the disk array controller begins flushing write cache content to disk media. The setting is specified as the percentage of total available cache that can contain write data before flushing begins. The cache flush threshold can be set independently for each controller. Note that available cache is reduced by half with cache mirroring enabled. For example, if the controller has 256 Mbytes of cache and cache mirroring is enabled, only 128 Mbytes of cache is available for each controller. The remaining 128 Mbytes is being used to mirror the other controller. See Figure 43. Performance Impact: This setting impacts write I/O performance by increasing or decreasing the number of write cache hits. (A "hit" is an I/O that can be serviced from cache rather than accessing the disk media.) A higher cache threshold value increases the amount of write data that will be stored in cache before flushing to the disk is initiated. This increases the possibility that subsequent writes may be serviced from cache rather than accessing the disk, reducing the number of writes to the disk media. This improves performance for write I/Os. However, a higher value reduces the amount of cache available for reads, which may reduce the performance for read I/Os. Lowering the cache threshold value has the opposite effect. Because less memory is used for write cache before flushing begins, the number of hits on write cache goes down with a subsequent decrease in write performance. However, read performance may improve because more memory is available for read cache. 132