Re: [PATCH, 3.7-rc7, RESEND] fs: revert commit bbdd6808 to fallocateUAPI

[Howard Chu]

Dave Chinner wrote:
> On Fri, Dec 07, 2012 at 03:25:53PM -0800, Howard Chu wrote:
> > I have to agree that, if this is going to be an ext4-specific
> > feature, then it can just be implemented via an ext4-specific ioctl
> > and be done with it. But I'm not convinced this should be an
> > ext4-specific feature.
> >
> > As for "fix the problem properly" - you're fixing the wrong problem.
> > This type of feature is important to me, not just because of the
> > performance issue. As has already been pointed out, the performance
> > difference may even be negligible.
> >
> > But on SSDs, the issue is write endurance. The whole point of
> > preallocating a file is to avoid doing incremental metadata updates.
> > Particularly when each of those 1-bit status updates costs entire
> > blocks, and gratuitously shortens the life of the media. The fact
> > that avoiding the unnecessary wear and tear may also yield a
> > performance boost is just icing on the cake. (And if the perf boost
> > is over a factor of 2:1 that's some pretty damn good icing.)
>
> That's a filesystem implementation specific problem, not a generic
> fallocate() or unwritten extent conversion problem.

> Besides, ext4 doesn't write back every metadata modification that is
> made - they are aggregated in memory and only written when the
> journal is full or the metadata ages out. Hence unwritten extent
> conversion has very little impact on the amount of writes that are
> done to the flash because it is vastly dominated by the data writes.
>
> Similarly, in XFS you might see a few thousand or tens of thousands
> of metadata blocks get written once every 30s under such a random
> write workload, but each metadata block might have gone through a
> million changes in memory since the last time it was written.
> Indeed, in that 30s, there would have been a few million random data
> writes so the metadata writes are well and truly lost in the
> noise...

That's only true if write caching is allowed. If you have a transactional 
database running, it's syncing every transaction to media.

--
  -- Howard Chu
  CTO, Symas Corp.           http://www.symas.com
  Director, Highland Sun     http://highlandsun.com/hyc/
  Chief Architect, OpenLDAP  http://www.openldap.org/project/
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