HP Surestore Disk Array 12h - And FC60, Advanced User'S Manual, Page: 251 / 466

Managing Disk Array Capacity
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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 87 .
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.
Table 31 Performance Impact of Configuration Settings (cont’d)