Re: [GENERAL] full_page_writes = off? - Mailing list pgsql-docs

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.

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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
>
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--
  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. +
Index: doc/src/sgml/wal.sgml
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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>