RE: Db file scattered reads vs. direct path reads
Date: Fri, 22 Feb 2008 12:48:20 -0800
Message-ID: <578806DDE66A3A45916740EB73C6982ADB05AF19B5@M1EXCHANGE01.mmi.local>
the old system is on ufs, the new is vxfs.
I've narrowed it down to the difference in synchronous disk reads. the async reads on the new system are much faster. the synchronous disk reads are slower.
From: Finn Jorgensen [mailto:finn.oracledba_at_gmail.com] Sent: Friday, February 22, 2008 7:01 AM
To: Josh Collier
Cc: oracle-l_at_freelists.org
Subject: Re: Db file scattered reads vs. direct path reads
What type of filesystem are you on? What differences are there between the old system and the new apart from the storage array?
Finn
On 2/20/08, Josh Collier <Josh.Collier_at_banfield.net<mailto:Josh.Collier_at_banfield.net>> wrote: Hi,
I've got a conundrum. We've got a new disk system. When I full scan a table with the nocache hint the trace files record lots of reads like this
WAIT #1: nam='db file scattered read' ela= 38471 file#=777 block#=238789 blocks=64 obj#=2805234 tim=359927798919
And the query takes 11 minutes for a 19gb table, much slower than our old system
When I enforce direct path reads with the following parameter :alter session set "_serial_direct_read" = true
The trace files record lots of reads like this
WAIT #1: nam='direct path read' ela= 5 file number=777 first dba=238789 block cnt=64 obj#=2805234 tim=343398354853
And the query returns in 3 minutes
What would cause the scattered reads to be so much slower than the direct path reads?
I've enabled direct io on the db and verified it via truss.
Version is 10.2.0.1<http://10.2.0.1/>
Solaris 10
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