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

[Howard Chu <hyc@symas.com>]

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/