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From: Bruce Momjian <bruce@momjian.us>
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Subject: Re: SSD + RAID
In-Reply-To: <alpine.DEB.2.00.0912081421120.25000@aragorn.flymine.org>
To: Matthew Wakeling <matthew@flymine.org>
Date: Sat, 20 Feb 2010 13:28:55 -0500 (EST)
CC: Greg Smith <greg@2ndquadrant.com>, Laszlo Nagy <gandalf@shopzeus.com>,
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Matthew Wakeling wrote:
> On Fri, 13 Nov 2009, Greg Smith wrote:
> > In order for a drive to work reliably for database use such as for 
> > PostgreSQL, it cannot have a volatile write cache.  You either need a write 
> > cache with a battery backup (and a UPS doesn't count), or to turn the cache 
> > off.  The SSD performance figures you've been looking at are with the drive's 
> > write cache turned on, which means they're completely fictitious and 
> > exaggerated upwards for your purposes.  In the real world, that will result 
> > in database corruption after a crash one day.
> 
> Seagate are claiming to be on the ball with this one.
> 
> http://www.theregister.co.uk/2009/12/08/seagate_pulsar_ssd/

I have updated our documentation to mention that even SSD drives often
have volatile write-back caches.  Patch attached and applied.

-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com
  PG East:  http://www.enterprisedb.com/community/nav-pg-east-2010.do
  + If your life is a hard drive, Christ can be your backup. +

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Index: doc/src/sgml/wal.sgml
===================================================================
RCS file: /cvsroot/pgsql/doc/src/sgml/wal.sgml,v
retrieving revision 1.61
diff -c -c -r1.61 wal.sgml
*** doc/src/sgml/wal.sgml	3 Feb 2010 17:25:06 -0000	1.61
--- doc/src/sgml/wal.sgml	20 Feb 2010 18:26:40 -0000
***************
*** 59,65 ****
     same concerns about data loss exist for write-back drive caches as
     exist for disk controller caches.  Consumer-grade IDE and SATA drives are
     particularly likely to have write-back caches that will not survive a
!    power failure.  To check write caching on <productname>Linux</> use
     <command>hdparm -I</>;  it is enabled if there is a <literal>*</> next
     to <literal>Write cache</>; <command>hdparm -W</> to turn off
     write caching.  On <productname>FreeBSD</> use
--- 59,66 ----
     same concerns about data loss exist for write-back drive caches as
     exist for disk controller caches.  Consumer-grade IDE and SATA drives are
     particularly likely to have write-back caches that will not survive a
!    power failure.  Many solid-state drives also have volatile write-back
!    caches.  To check write caching on <productname>Linux</> use
     <command>hdparm -I</>;  it is enabled if there is a <literal>*</> next
     to <literal>Write cache</>; <command>hdparm -W</> to turn off
     write caching.  On <productname>FreeBSD</> use

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