Topic: Performance & Tuning >> Hit Ratio of Database Buffer Cache
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| Title: Hit Ratio of Database Buffer Cache |
| Navi |
Posted: Jun 13, 2007 08:09:00 AM |
Total Post: 60
Joined: Jun, 2007
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Hi,
Anyone please answer this question
Why 99%+ Database Buffer Cache hit ratio is not OK?
Regards,
Naveen |
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 Ratnaker |
| Posted: Jul 03, 2007 02:23:36 AM |  |
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Total Post: 136
Joined: Apr, 2007
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hi Naveen,
This is the percentage of requests
for a particular block which are satisfied within the cache without the need for physical IO.
Although historically known as one of the most important statistics, the buffer cache hit ratio
can sometimes be misleading. A high (e.g. 99%) cache hit ratio normally indicates the cache
is adequately sized, however this may not be the case in all circumstances. For example,
frequently executed SQL statements which repeatedly refer to a small number of buffers via
indexed lookups can skew the buffer gets statistic. When these blocks are read, they are
placed at the most recently used (MRU) end of the buffer cache; iterative access to these
blocks can artificially inflate the cache hit ratio. This makes tuning the buffer cache a
challenging activity.
On some sites, it is possible to identify a too small buffer cache by the appearance of the
‘write complete waits’ event, which indicates that hot blocks (i.e. blocks which are still being
modified) are aging out of the cache while they are still needed; check the Wait events
section for evidence of this event.
Alternatively, a lower buffer cache hit ratio does not necessarily mean the cache is too small;
it may be that (potentially valid) full table scans are artificially reducing what is otherwise a
good hit ratio.
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