From: Bruce Momjian <bruce@momjian.us>
To: Greg Smith <gsmith@gregsmith.com>
Cc: PostgreSQL-documentation <pgsql-docs@postgresql.org>
Subject: Re: [GENERAL] full_page_writes = off?
Date: Mon, 10 Dec 2007 09:06:26 -0500 (EST)
Message-ID: <200712101406.lBAE6QU19627@momjian.us> (raw)
In-Reply-To: <Pine.GSO.4.64.0712021528290.2007@westnet.com>
I thought we had the disk write cache control documented for popular
operating systems, but I see we don't so I added the following patch to
our documentation.
---------------------------------------------------------------------------
Greg Smith wrote:
> On Sat, 1 Dec 2007, Tomasz Ostrowski wrote:
>
> > You can also use "hdparm -I" to check this - look for a "Write
> > caching" in "Commands/features" section. If it has a "*" in front
> > then it is enabled and dangerous.
>
> Right; using -I works with most Linux hdparm versions:
>
> # hdparm -V
> hdparm v6.6
> # hdparm -I /dev/hda | grep "Write cache"
> * Write cache
> # hdparm -W 0 /dev/hda
>
> /dev/hda:
> setting drive write-caching to 0 (off)
> # hdparm -I /dev/hda | grep "Write cache"
> Write cache
>
> While being able to check the state with -W only works in very recent
> ones.
>
> The best way to make this change permanent varies depending on your Linux
> distribution.
>
> Also: nowadays many SATA disks appear as SCSI devices like /dev/sda. In
> some cases I believe you can use a recent hdparm on them anyway, in others
> I've had to use sdparm instead. Several of the examples at
> http://sg.torque.net/sg/sdparm.html show how to manipulate the Write Cache
> Enabled (WCE) status similarly to the above on SCSI devices.
>
> > I don't know how to check it on BSD.
>
> In FreeBSD I believe you use atacontrol to check the settings, and you can
> make the changes permanent by fiddling with the /boot/device.hints file.
>
> --
> * Greg Smith gsmith@gregsmith.com http://www.gregsmith.com Baltimore, MD
>
> ---------------------------(end of broadcast)---------------------------
> TIP 4: Have you searched our list archives?
>
> http://archives.postgresql.org/
--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://postgres.enterprisedb.com
+ If your life is a hard drive, Christ can be your backup. +
Attachments:/rtmp/diff (text/x-diff, 1.5K) download
Index: doc/src/sgml/wal.sgml
===================================================================
RCS file: /cvsroot/pgsql/doc/src/sgml/wal.sgml,v
retrieving revision 1.46
diff -c -c -r1.46 wal.sgml
*** doc/src/sgml/wal.sgml 28 Nov 2007 15:42:31 -0000 1.46
--- doc/src/sgml/wal.sgml 10 Dec 2007 14:04:12 -0000
***************
*** 58,65 ****
while some are write-back, and the
same concerns about data loss exist for write-back drive caches as
exist for disk controller caches. Consumer-grade IDE drives are
! particularly likely to contain write-back caches that will not
! survive a power failure.
</para>
<para>
--- 58,74 ----
while some are write-back, and the
same concerns about data loss exist for write-back drive caches as
exist for disk controller caches. Consumer-grade IDE 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</> can to turn off
! write caching. On <productname>FreeBSD</> use
! <application>atacontrol</>. (For SCSI disks use <ulink
! url="http://sg.torque.net/sg/sdparm.html"><application>sdparm</></ulink>
! to turn off <literal>WCE</>.) On <productname>Windows</> write caching is
! controlled by <literal>My Computer\Open\{select disk
! drive}\Properties\Hardware\Properties\Policies\Enable write caching on
! the disk</>.
</para>
<para>
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-docs@postgresql.org
Cc: bruce@momjian.us, gsmith@gregsmith.com
Subject: Re: [GENERAL] full_page_writes = off?
In-Reply-To: <200712101406.lBAE6QU19627@momjian.us>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox