Date: Thu, 14 May 2015 12:29:04 -0400
From: Ravi Krishna <s.ravikrishna@aim.com>
To: bruce@momjian.us
Cc: tgl@sss.pgh.pa.us, pgsql-admin@postgresql.org
Message-Id: <14d5342006a-2108-312c5@webstg-m03.mail.aol.com>
In-Reply-To: <20150514162610.GB23739@momjian.us>
Subject: Re: WAL and O_DIRECT
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Thanks Bruce and Tom. That pretty much explains it.


-----Original Message-----
From: Bruce Momjian <bruce@momjian.us>
To: Ravi Krishna <s.ravikrishna@aim.com>
Cc: tgl <tgl@sss.pgh.pa.us>; pgsql-admin <pgsql-admin@postgresql.org>
Sent: Thu, May 14, 2015 12:26 pm
Subject: Re: [ADMIN] WAL and O_DIRECT


On Thu, May 14, 2015 at 12:07:04PM -0400, Ravi Krishna wrote:> >> However o=
urDB2 folks are raising a concern that PG WAL writes may not be crash safe,=
 unlesswe are using> >> write back technology in SAN or SSD , which we are =
using.>> > What's your point exactly?  If the underlying hardware does not =
providedurable writes, there's> > nothing PG (or DB2) can do to fix that.> =
> >Am I right in concluding that PG WAL writes without underlying h/w cachi=
ng isnot crash proof.> Fortunately these days caching is  ubiquitous in all=
 SSD/SANtechnology. Both Oracle and DB2 always open WAL> logs in O_DIRECT. =
Is thisthinking outdated with modern technology which caches writes. Wonder=
 whyOracle/DB2> are not making O_DIRECT optional. I am sure it will increas=
e thewrite performance.Basically, O_DIRECT writes through the OS catch dire=
ctlyto the storage.Postgres writes to the OS cache, then uses fsync() or an=
otherOS call toflush that OS write to the storage --- we just do it in twop=
arts.We turn off O_DIRECT for WAL writes because we know another processisg=
oing to read it soon, so in that case, we fall back the two-partsolutionof =
OS write and fsync-like OS call.--   Bruce Momjian  <bruce@momjian.us> http=
://momjian.us  EnterpriseDB                            http://enterprisedb.=
com  + Everyone has their own god. +


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