Messages in this thread | | | Date | Sat, 08 Dec 2012 05:52:30 -0800 | From | Howard Chu <> | Subject | Re: [PATCH, 3.7-rc7, RESEND] fs: revert commit bbdd6808 to fallocate UAPI |
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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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