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Data item
Cache Prefetch Efficiency
Version: 11.0 and later
Cache Refer and Reuse
Version: 11.0 and later
Cache Reuse
Version: 11.0 and later
Cache Reuse Dirty
Version: 11.0 and later
Cache Size
Version: 11.0 and later
User's Guide
Description
A percentage comparing the count of pages being reused in large I/O buffers (the
denominator) to the number of those pages that were ever referenced by Adaptive
Server. When a buffer is reused, all of the pages in it are counted as reused. Buffers are
reused when there are no free buffers in the pool to accept a new physical read from a
database device. The number of reused pages referenced by Adaptive Server divided
by the result of the number of pages per buffer multiplied by the number of reused
buffers indicates the effectiveness of large I/O fetches.
Regardless of how many buffer pools are configured in a named data cache, Adaptive
Server only uses two of them. It uses the 2K buffer pool and the pool configured with
the largest-sized buffers. Prefetch effectiveness does not apply to the 2K buffer pool,
since 2K grabs are not considered large I/O. Therefore, prefetch effectiveness applies
to the largest buffer pool in the cache. For example, if a data cache has pools of buffers
sized 2K, 8K, and 16K, the 8K pool is not used, and this metric reflects the
effectiveness of large I/O in the 16K buffer pool.
If the ratio is large, then prefetching is effective; otherwise, prefetching is not
providing much benefit. This may suggest that a buffer pool should be eliminated (or
it may imply that a clustered index on some table is fragmented, and that the index
should be dropped and re-created).
Type: Result
The number of pages in buffers that were both referenced and reused. This count is
employed when determining the efficiency of prefetching buffers (see Cache Prefetch
Efficiency). This data item, unlike Cache Prefetch Efficiency, includes activity in the
default 2K buffer pool.
See Cache Prefetch Efficiency for a definition of buffer reuse.
Type: Result
The number of pages in buffers that were reused. A large value indicates a high rate of
turnover (of buffers in this memory pool), and suggests that the pool may be too small.
A zero value suggests that this memory pool may be too large. This data item, unlike
Cache Prefetch Efficiency, includes activity in the default 2K buffer pool.
See Cache Prefetch Efficiency for a definition of buffer reuse.
Type: Result
The number of times that a buffer that was reused had changes that needed to be
written. A non-zero value indicates that the wash size is too small.
See Cache Prefetch Efficiency for a definition of buffer reuse.
Type: Result
The size of a data cache, in megabytes.
Type: Result
APPENDIX A
Data Items
105
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