[PATCH 05/18] xfs: Use preallocation for inodes with extsz hints

[Mark Tinguely]

On 04/13/12 07:10, Dave Chinner wrote:
> From: Dave Chinner<dchinner at redhat.com>
>
> xfstest 229 exposes a problem with buffered IO, delayed allocation
> and extent size hints. That is when we do delayed allocation during
> buffered IO, we reserve space for the extent size hint alignment and
> allocate the physical space to align the extent, but we do not zero
> the regions of the extent that aren't written by the write(2)
> syscall. The result is that we expose stale data in unwritten
> regions of the extent size hints.
>
> There are two ways to fix this. The first is to detect that we are
> doing unaligned writes, check if there is already a mapping or data
> over the extent size hint range, and if not zero the page cache
> first before then doing the real write. This can be very expensive
> for large extent size hints, especially if the subsequent writes
> fill then entire extent size before the data is written to disk.
>
> The second, and simpler way, is simply to turn off delayed
> allocation when the extent size hint is set and use preallocation
> instead. This results in unwritten extents being laid down on disk
> and so only the written portions will be converted. This matches the
> behaviour for direct IO, and will also work for the real time
> device. The disadvantage of this approach is that for small extent
> size hints we can get file fragmentation, but in general extent size
> hints are fairly large (e.g. stripe width sized) so this isn't a big
> deal.
>
> Implement the second approach as it is simple and effective.
>
> Signed-off-by: Dave Chinner<dchinner at redhat.com>
> ---


Looks good.

Reviewed-by: Mark Tinguely <tinguely at sgi.com>