From: Ravi Krishna <s.ravikrishna@aim.com>
To: tgl@sss.pgh.pa.us
Cc: pgsql-admin@postgresql.org
Subject: Re: WAL and O_DIRECT
Date: Thu, 14 May 2015 11:32:12 -0400
Message-ID: <14d530df35f-2108-30ce7@webstg-m03.mail.aol.com> (raw)
In-Reply-To: <20588.1431616292@sss.pgh.pa.us>
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Aha that pretty much explains it. Yes we are using streaming replication.
However our DB2 folks are raising a concern that PG WAL writes may not be crash safe, unless we are using write back technology in SAN or SSD , which we are using.
-----Original Message-----
From: Tom Lane <tgl@sss.pgh.pa.us>
To: Ravi Krishna <s.ravikrishna@aim.com>
Cc: pgsql-admin <pgsql-admin@postgresql.org>
Sent: Thu, May 14, 2015 11:11 am
Subject: Re: [ADMIN] WAL and O_DIRECT
Ravi Krishna <s.ravikrishna@aim.com> writes:
> Why is O_DIRECT not used,
despite the documentation mentioning otherwise?
You've not shown us all your
settings, but this comment in xlog.c might
explain it:
* Optimize
writes by bypassing kernel cache with O_DIRECT when using
* O_SYNC/O_FSYNC
and O_DSYNC. But only if archiving and streaming are
* disabled,
otherwise the archive command or walsender process will read
* the WAL
soon after writing it, which is guaranteed to cause a physical
* read if
we bypassed the kernel cache. We also skip the
*
posix_fadvise(POSIX_FADV_DONTNEED) call in XLogFileClose() for the same
*
reason.
regards, tom lane
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Subject: Re: WAL and O_DIRECT
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