Date: Thu, 28 May 2015 10:27:13 -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: <20150528142713.GA18209@momjian.us>
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On Thu, May 28, 2015 at 01:37:48PM +0000, Graeme B. Bell wrote:
> Hi Bruce
> 
> I'm *extremely* certain of what I say when I say WB+BBU=good and direct WT=bad.

It is my understanding that write-through is always safe as it writes
through to the layer below and waits for acnoledgement.  Write-back
doesn't, so when you say:

> WT should be OK with e.g. Intel SSDs.

I assume you mean Write-Back is OK because the drive has a BBU, while
Write-Through is always safe.

You also say:

> >> - WT with most SSDs will likely corrupt your postgres database the
> >> first time you lose power. (on all the drives I've tested)

which contradicts what you said above, and I assume you mean write-back
here.

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

  + Everyone has their own god. +


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