HP Surestore Disk Array 12h - And FC60, Руководство для продвинутых пользователей, Страница: 252 / 466

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Managing Disk Array Capacity
Setting: Cache flush limit (default 100%)
Function:
Determines how much data will remain in write cache when flushing stops. It is
expressed as a percentage of the cache flush threshold. For optimum performance this value is
set to 100% by default. This ensures that the entire amount of cache specified by the cache flush
threshold will contain write cache data, increasing the number of write cache hits. The cache
flush threshold can be set independently for each controller.
A value of 100% percent can be used for this setting because the cache flushing algorithm
typically overshoots a bit in both directions. Assuming a cache flush threshold of 80% (default),
the write cache contents will actually rise slightly past the threshold before flushing begins - to
82% for example. The cache flush limit of 100% (default) indicates that flushing should stop
when the cache contents reaches 80%, but the flushing continues until the cache reaches a
slightly lower level - 78% for example. This has the effect of maintaining a constant 80% level of
data in write cache. See
Figure 87 .
Performance Impact: A high value for the flush limit increases the amount of data remaining in
write cache at the end of the flush. This improves the possibility of hits on write cache. Because
less data is written to the disk during each flush cycle, data flushing will occur more often,
increasing disk activity. In most situations this value should be left at the default value of 100%.
A lower value reduces the amount of write data remaining in cache, thereby reducing the number
of write hits. More data is flushed during each cycle, reducing the number of flushes. There will
be fewer cache flush operations, but each one will last longer.
Setting:
Cache page size
Function:
Sets the number of blocks transferred into cache by the controller at one time.
Performance Impact: A larger cache block size increases the probability of data near the
accessed block also being available in cache. However, a larger cache block size also fills up the
cache sooner. A small cache block size should be used for systems that require transaction-
processing requests, or I/O streams that are typically small and random. Larger cache blocks
should be used for large I/O, sequential, high bandwidth applications.
Table 31 Performance Impact of Configuration Settings (cont’d)