Re: OCFS2, blockdev and readahead
- From: Jeremy Schneider <jeremy.schneider@xxxxxxxxxxxxxx>
- To: vbarac@xxxxxxxxxxxx
- Date: Sun, 08 Feb 2009 15:05:39 -0600
Vladimir Barac wrote:
Oracle 10.2.0.2, OCFS2, single instance database, NetApp FCP disks
filesystemio_options=setall
db_file_multiblock_read_count=128
Do we need to set readahead to greater than 0 for OCFS2? What would be
ideal value? How to measure impact – positive/negative?
Storage admins would prefered "blockdev –-setra" to be set to maximum
value, to speed up filesystem backups. On the other hand, since oracle
will use O_DIRECT on OCFS2, I believe we don't need readahead to be set.
It's (as always) a little application dependent, although my experience
has been that very few databases benefit from read-ahead. (Just some
warehouses/datamarts.) NetApp has published best practices and they have
a decent paragraph about this - as of May 2008 they generally recommend
not using readahead.
http://www.netapp.com/us/library/technical-reports/tr-3369.html
These read ahead recommendations would apply the same on OCFS2 or ASM or
NFS or a regular filesystem. Even though read ahead doesn't usually kick
in right away anyway, few databases perform many large sequential reads
and it would probably start doing read ahead right when Oracle stops
reading data.
Furthermore, you certainly shouldn't tune your storage system to perform
fast backups and do everything else a little more slowly. :) And if
someday you move toward incremental or even updated image copy backups,
then the readahead won't benefit at all for backups.
-Jeremy
--
Jeremy Schneider
Chicago, IL
http://www.ardentperf.com
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