Re: [PATCH 1/5] fallocate() implementation in i86, x86_64 and powerpc

David Chinner
2007-05-10T01:00:05Z

On Wed, May 09, 2007 at 09:31:02PM +0530, Amit K. Arora wrote:
> I have the updated patches ready which take care of Andrew's comments.
> Will run some tests and post them soon.
>
> But, before submitting these patches, I think it will be better to finalize
> on certain things which might be worth some discussion here:
>
> 1) Should the file size change when preallocation is done beyond EOF ?
> - Andreas and Chris Wedgwood are in favor of not changing the
> file size in this case. I also tend to agree with them. Does anyone
> has an argument in favor of changing the filesize ?
> If not, I will remove the code which changes the filesize, before I
> resubmit the concerned ext4 patch.

I think there needs to be both. If we don't have a mechanism to
atomically change the file size with the preallocation, then
applications that use stat() to work out if they need to preallocate
more space will end up racing.

> 2) For FA_UNALLOCATE mode, should the file system allow unallocation
> of normal (non-preallocated) blocks (blocks allocated via
> regular write/truncate operations) also (i.e. work as punch()) ?

Yes. That is the current XFS implementation for XFS_IOC_UNRESVSP, and
what i did for FA_UNALLOCATE as well.

> - Though FA_UNALLOCATE mode is yet to be implemented on ext4, still
> we need to finalize on the convention here as a general guideline
> to all the filesystems that implement fallocate.
>
> 3) If above is true, the file size will need to be changed
> for "unallocation" when block holding the EOF gets unallocated.

No - we punch a hole. If you want the filesize to change, then
you use ftruncate() to remove the blocks at EOF and change the
file size atomically.

> 4) Should we update mtime & ctime on a successfull allocation/
> unallocation ?
> - David Chinner raised this question in following post:
> http://lkml.org/lkml/2007/4/29/407
> I think it makes sense to update the [mc]time for a successfull
> preallocation/unallocation. Does anyone feel otherwise ?
> It will be interesting to know how XFS behaves currently. Does XFS
> update [mc]time for preallocation ?

No, XFS does *not* update a/m/ctime on prealloc/punch unless the file size
changes. If the filesize changes, it behaves exactly the same way that
ftruncate() behaves.

Cheers,

Dave.
--
Dave Chinner
Principal Engineer
SGI Australian Software Group