Re: WAL and O_DIRECT

[Ravi Krishna <s.ravikrishna@xxxxxxx>]

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@xxxxxxxxxxxxx>
To: Ravi Krishna <s.ravikrishna@xxxxxxx>
Cc: pgsql-admin <pgsql-admin@xxxxxxxxxxxxxx>
Sent: Thu, May 14, 2015 11:11 am
Subject: Re: WAL and O_DIRECT

Ravi Krishna <s.ravikrishna@xxxxxxx> 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