From: Bruce Momjian <bruce@momjian.us>
Message-Id: <200712101406.lBAE6QU19627@momjian.us>
Subject: Re: [GENERAL] full_page_writes = off?
In-Reply-To: <Pine.GSO.4.64.0712021528290.2007@westnet.com>
To: Greg Smith <gsmith@gregsmith.com>
Date: Mon, 10 Dec 2007 09:06:26 -0500 (EST)
CC: PostgreSQL-documentation <pgsql-docs@postgresql.org>
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="ELM1197295586-25695-0_"
Content-Transfer-Encoding: 7bit


--ELM1197295586-25695-0_
Content-Transfer-Encoding: 7bit
Content-Type: text/plain; charset="US-ASCII"


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. +

--ELM1197295586-25695-0_
Content-Transfer-Encoding: 7bit
Content-Type: text/x-diff
Content-Disposition: inline; filename="/rtmp/diff"

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>

--ELM1197295586-25695-0_--