* Bruce Momjian <pgman@candle.pha.pa.us> [010502 14:01] wrote:
> I was talking to a Linux user yesterday, and he said that performance
> using the xfs file system is pretty bad. He believes it has to do with
> the fact that fsync() on log-based file systems requires more writes.
>
> With a standard BSD/ext2 file system, WAL writes can stay on the same
> cylinder to perform fsync. Is that true of log-based file systems?
>
> I know xfs and reiser are both log based. Do we need to be concerned
> about PostgreSQL performance on these file systems? I use BSD FFS with
> soft updates here, so it doesn't affect me.
The "problem" with log based filesystems is that they most likely
do not know the consequences of a write so an fsync on a file may
require double writing to both the log and the "real" portion of
the disk. They can also exhibit the problem that an fsync may
cause all pending writes to require scheduling unless the log is
constructed on the fly rather than incrementally.
There was also the problem that was brought up recently that
certain versions (maybe all?) of Linux perform fsync() in a very
non-optimal manner, if the user is able to use the O_FSYNC option
rather than fsync he may see a performance increase.
But his guess is probably nearly as good as mine. :)
--
-Alfred Perlstein - [alfred@freebsd.org]
http://www.egr.unlv.edu/~slumos/on-netbsd.html
