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Re: BBU Cache vs. spindles

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>
Cc: Greg Smith <greg(at)2ndquadrant(dot)com>, jd(at)commandprompt(dot)com, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>, Steve Crawford <scrawford(at)pinpointresearch(dot)com>, pgsql-performance(at)postgresql(dot)org, Ben Chobot <bench(at)silentmedia(dot)com>
Subject: Re: BBU Cache vs. spindles
Date: 2010-12-01 03:13:12
Message-ID: 201012010313.oB13DCu03542@momjian.us (view raw or flat)
Thread:
Lists: pgsql-performancepgsql-www
Kevin Grittner wrote:
> Greg Smith <greg(at)2ndquadrant(dot)com> wrote:
>  
> > I think Kevin's point here may be that if your fsync isn't
> > reliable, you're always in trouble.  But if your fsync is good,
> > even torn pages should be repairable by the deltas written to the
> > WAL
>  
> I was actually just arguing that a BBU doesn't eliminate a risk
> here; if there is a risk with production-quality disk drives, there
> is a risk with a controller with a BBU cache.  The BBU cache just
> tends to reduce the window of time in which corruption can occur.  I
> wasn't too sure of *why* there was a risk, but Tom's post cleared
> that up.
>  
> I wonder why we need to expose this GUC at all -- perhaps it should
> be off when fsync is off and on otherwise?  Leaving it on without
> fsync is just harming performance for not much benefit, and turning
> it off with fsync seems to be saying that you are willing to
> tolerate a known risk of database corruption, just not quite so much
> as you have without fsync.  In reality it seems most likely to be a
> mistake, either way.

According to our docs, and my submitted patch, if you are using ZFS then
you can turn off full-page writes, so full-page writes are still useful.

-- 
  Bruce Momjian  <bruce(at)momjian(dot)us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +

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