From: Bruce Momjian <bruce@momjian.us>
To: Matthew Wakeling <matthew@flymine.org>
Cc: Greg Smith <greg@2ndquadrant.com>
Cc: Laszlo Nagy <gandalf@shopzeus.com>
Cc: pgsql-performance <pgsql-performance@postgresql.org>
Subject: Re: SSD + RAID
Date: Sat, 20 Feb 2010 13:28:55 -0500 (EST)
Message-ID: <201002201828.o1KIStQ12830@momjian.us> (raw)
In-Reply-To: <alpine.DEB.2.00.0912081421120.25000@aragorn.flymine.org>
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. +
Attachments:/rtmp/diff (text/x-diff, 1.4K) download
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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To: pgsql-performance@postgresql.org
Cc: bruce@momjian.us, matthew@flymine.org, greg@2ndquadrant.com, gandalf@shopzeus.com
Subject: Re: SSD + RAID
In-Reply-To: <201002201828.o1KIStQ12830@momjian.us>
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