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Oracle performance question for AWR reports
Below is an Oracle performance question for AWR reports from Oracle Specialists Community:
I got 2% physical reads in the field of Physical Reads/Total Read ( slide 8 ) from Oracle AWR reports.
- Does it mean that 98% of reads came from Oracle SGA?
- From Oracle perspective does physical read could be also reads from the cache of the DMX?
- In such case, do you think VFcache is irrelevant?
zhaos2
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November 4th, 2012 23:00
An answer from Oracle Specialist:
When Oracle needs to access a block – it is logical I/O.
Oracle looks it up in the SGA, if it doesn’t find the block cached, it issues a physical I/O to the O/S.
The I/O completes when the O/S returns the I/O – from O/S cache (rare – usually Oracle does direct reads, bypassing the O/S cache), or from SAN/NAS (whether cached or not in the storge).
So:
I would always look at AWR “Top 5 Wait Events” to have a felling where most of the database time is being spent.
If the system is not I/O bound, no need for faster I/O.
It seems from a quick scan of the ppt – they have low latency I/O + not a lot of IOPS + not a lot of bandwidth - no indication