Radoulov,
The caching in the SGA understands your data usage patterns through the LRU
algorithms and will have cached all of the best data. The FS cache, if you dump
it out, will look a lot more like white noise with few discernable patterns.
The SAN cache even more so. The more single block reads you have, the more like
white noise it all looks. The liklihood of there being a cache hit in the FS or
SAN cache is relatively low. The advantage of direct path reads significantly
outweights the advantage of both of those caches. It is worth noting in that on
most SAN caches, if you specify that the LUN is for a database it will disable
read-ahead to pre-populate the cache as it understands that it is not the best
use of the cache (the general rule is that SAN cache should be reserved
exclusively for writes when the SAN is used for the database.)
Note that these statements are generalisation, and that there may be cases
where your assertion is true but they will be an edge case and I would
recommend that you have a provable scenario to justify running in that
configuration.
Neil Chandler
Database Guy.
________________________________
From: oracle-l-bounce@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx> on behalf
of Radoulov, Dimitre <cichomitiko@xxxxxxxxx>
Sent: 31 October 2018 07:20
To: Andrew Kerber
Cc: lkaing@xxxxxxxxx; contact@xxxxxxxx; Oracle-L Group
Subject: Re: Storage choice for Oracle database on VMware
Thank you all for the valuable input!
what is the problem with direct I/O? You should never run an Oracle database
through page cache anyway :)
On 31 Oct 2018, at 7:20 am, Stefan Koehler--
<contact@xxxxxxxx<mailto:contact@xxxxxxxx>> wrote:
Hello Dimitre,
what is the problem with direct I/O? You should never run an Oracle database
through page cache anyway :)
I would go with tweaked XFS (e.g. "nobarrier" as this information is usually
not passed through correctly with VMDKs on VMFS, etc.) if it is just one
single instance in this VM.
Best Regards
Stefan Koehler
Independent Oracle performance consultant and researcher
Website: http://www.soocs.de
Twitter: @OracleSK
"Radoulov, Dimitre" <cichomitiko@xxxxxxxxx<mailto:cichomitiko@xxxxxxxxx>>--
hat am 30. Oktober 2018 um 19:12 geschrieben:
Thank you Chris, Matthew and Niall,
so the question is if performancewise ASM is worth it.
With the default Oracle database settings the I/O on XFS would be
synchronous, right?
And if I understand correctly Note 1987437.1, on Linux you cannot enable
async I/O without turning on direct I/O too.
Regards
Dimitre
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