[PATCH 04/60] xfs: don't use speculative prealloc for small files

[Mark Tinguely]

On 06/18/13 23:50, Dave Chinner wrote:
> From: Dave Chinner<dchinner at redhat.com>
>
> Dedicated small file workloads have been seeing significant free
> space fragmentation causing premature inode allocation failure
> when large inode sizes are in use. A particular test case showed
> that a workload that runs to a real ENOSPC on 256 byte inodes would
> fail inode allocation with ENOSPC about about 80% full with 512 byte
> inodes, and at about 50% full with 1024 byte inodes.
>
> The same workload, when run with -o allocsize=4096 on 1024 byte
> inodes would run to being 100% full before giving ENOSPC. That is,
> no freespace fragmentation at all.
>
> The issue was caused by the specific IO pattern the application had
> - the framework it was using did not support direct IO, and so it
> was emulating it by using fadvise(DONT_NEED). The result was that
> the data was getting written back before the speculative prealloc
> had been trimmed from memory by the close(), and so small single
> block files were being allocated with 2 blocks, and then having one
> truncated away. The result was lots of small 4k free space extents,
> and hence each new 8k allocation would take another 8k from
> contiguous free space and turn it into 4k of allocated space and 4k
> of free space.
>
> Hence inode allocation, which requires contiguous, aligned
> allocation of 16k (256 byte inodes), 32k (512 byte inodes) or 64k
> (1024 byte inodes) can fail to find sufficiently large freespace and
> hence fail while there is still lots of free space available.
>
> There's a simple fix for this, and one that has precendence in the
> allocator code already - don't do speculative allocation unless the
> size of the file is larger than a certain size. In this case, that
> size is the minimum default preallocation size:
> mp->m_writeio_blocks. And to keep with the concept of being nice to
> people when the files are still relatively small, cap the prealloc
> to mp->m_writeio_blocks until the file goes over a stripe unit is
> size, at which point we'll fall back to the current behaviour based
> on the last extent size.
>
> This will effectively turn off speculative prealloc for very small
> files, keep preallocation low for small files, and behave as it
> currently does for any file larger than a stripe unit. This
> completely avoids the freespace fragmentation problem this
> particular IO pattern was causing.
>
> Signed-off-by: Dave Chinner<dchinner at redhat.com>
> ---

I agree with Brian, it looks good.

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