Michael Renner wrote:
> Hi,
>
> the comment WRT WAL recovery and FS journals [1] is a bit misleading in
> it's current form.
>
> First, none of the general purpose filesystems I've seen so far do data
> journalling per default, since it's a huge performance penalty, even for
> non-RDBMS workloads. The feature you talk about is ext3 specific (and
> should be pointed out as such) and only disables write ordering, meaning
> that metadata and file content updates are not synchronized.
You are right that my docs were misleading. I have improved them by
mentioning that it is _data_ flush that as part of journalling that can
be a problem, and documented that the mount option listed is
ext3-specific, not linux-specific.
Updated docs attached. Please let me know if I can improve it some
more.
--
Bruce Momjian <bruce@momjian.us> http://momjian.us
EnterpriseDB http://enterprisedb.com
+ If your life is a hard drive, Christ can be your backup. +
Index: doc/src/sgml/wal.sgml
===================================================================
RCS file: /cvsroot/pgsql/doc/src/sgml/wal.sgml,v
retrieving revision 1.54
diff -c -c -r1.54 wal.sgml
*** doc/src/sgml/wal.sgml 6 Dec 2008 21:34:27 -0000 1.54
--- doc/src/sgml/wal.sgml 10 Dec 2008 11:04:08 -0000
***************
*** 139,151 ****
<para>
Because <acronym>WAL</acronym> restores database file
contents after a crash, it is not necessary to use a
! journaled filesystem; in fact, journaling overhead can
! reduce performance. For best performance, turn off
! <emphasis>data</emphasis> journaling as a filesystem mount
! option, e.g. use <literal>data=writeback</> on Linux.
! Meta-data journaling (e.g. file creation and directory
! modification) is still desirable for faster rebooting after
! a crash.
</para>
</tip>
--- 139,151 ----
<para>
Because <acronym>WAL</acronym> restores database file
contents after a crash, it is not necessary to use a
! journaled filesystem for reliability. In fact, journaling
! overhead can reduce performance, especially if journaling
! causes file system <emphasis>data</emphasis> to be flushed
! to disk. Fortunately, data flushing during journaling can
! often be disabled with a filesystem mount option, e.g.
! <literal>data=writeback</> on a Linux ext3 file system.
! Journaled file systems do improve boot speed after a crash.
</para>
</tip>