* Bruce Momjian <pgman@candle.pha.pa.us> [010502 15:20] wrote:
> > 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.
>
> Yes, this double-writing is a problem. Suppose you have your WAL on a
> separate drive. You can fsync() WAL with zero head movement. With a
> log based file system, you need two head movements, so you have gone
> from zero movements to two.
It may be worse depending on how the filesystem actually does
journalling. I wonder if an fsync() may cause ALL pending
meta-data to be updated (even metadata not related to the
postgresql files).
Do you know if reiser or xfs have this problem?
--
-Alfred Perlstein - [alfred@freebsd.org]
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