Date: Wed, 27 May 2015 19:20:59 -0400
From: Bruce Momjian <bruce@momjian.us>
To: "Graeme B. Bell" <grb@skogoglandskap.no>
Cc: "pgsql-admin@postgresql.org" <pgsql-admin@postgresql.org>
Subject: Re: raid writethrough mode (WT), ssds and your DB. (was
 Performances issues with SSD volume ?)
Message-ID: <20150527232059.GA7962@momjian.us>
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On Thu, May 21, 2015 at 11:21:49AM +0000, Graeme B. Bell wrote:
> > Not using your raid controllers write cache then?  Not sure just how
> > important that is with SSDs these days, but if you've got a BBU set
> > it to "WriteBack". Also change "Cache if Bad BBU" to "No Write Cache
> > if Bad BBU" if you do that.
>
> I did quite a few tests with WB and WT last year.
>
> - WT should be OK with e.g. Intel SSDs.  From memory I saw write
> performance gains of about 20-30% with Crucial M500/M550 writes on a
> Dell H710 RAID controller. BUT that controller didn't have WT fastpath
> though which is absolutely essential to see substantial gains WT. I
> expect with WT and a fastpath enabled RAID you'd see much higher
> numbers, e.g. 100%+ higher IOPS.
>
> (So, if you don't have fastpath on your controller, you might as
> well plan to leave WB on and just buy cheaper SSD drives rather than
> expensive ones - the raid controller will be your choke point for
> performance on WT and it's a source of risk).
>
> - WT with most SSDs will likely corrupt your postgres database the
> first time you lose power. (on all the drives I've tested)
>
> - WB is the only safe option unless you have done lots of plug pull
> tests on a drive that is guaranteed to protect data "in flight" during
> power loss (Intel disks + maybe the new samsung pcie).

I think you have WT (write-through) and WB (write-back) reversed above.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + Everyone has their own god. +


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