Re: WAL and O_DIRECT

Thanks Bruce and Tom. That pretty much explains it.

-----Original Message-----
From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Ravi Krishna <s(dot)ravikrishna(at)aim(dot)com>
Cc: tgl <tgl(at)sss(dot)pgh(dot)pa(dot)us>; pgsql-admin <pgsql-admin(at)postgresql(dot)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 ourDB2 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 caching 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 Is thisthinking outdated with modern technology which caches writes.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 increase thewrite performance.Basically, O_DIRECT writes through the OS catch directlyto the storage.Postgres writes to the OS cache, then uses fsync() or anotherOS call toflush that OS write to the storage --- we just do it in twoparts.We turn off O_DIRECT for WAL writes because we know another processisgoing to read it soon, so in that case, we fall back the two-partsolutionof OS write and fsync-like OS call.-- Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us EnterpriseDB http://enterprisedb.com + Everyone has their own god. +