Re: New Linux xfs/reiser file systems

> 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.

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