[PATCH 07/34] xfs: don't truncate prealloc from frequently accessed inodes

[Christoph Hellwig]

On Tue, Dec 21, 2010 at 06:29:03PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner at redhat.com>
> 
> A long standing problem for streaming write?? through the NFS server
> has been that the NFS server opens and closes file descriptors on an
> inode for every write. The result of this behaviour is that the
> ->release() function is called on every close and that results in
> XFS truncating speculative preallocation beyond the EOF.  This has
> an adverse effect on file layout when multiple files are being
> written at the same time - they interleave their extents and can
> result in severe fragmentation.
> 
> To avoid this problem, keep a count of the number of ->release calls
> made on an inode. For most cases, an inode is only going to be opened
> once for writing and then closed again during it's lifetime in
> cache. Hence if there are multiple ->release calls, there is a good
> chance that the inode is being accessed by the NFS server. Hence
> count up every time ->release is called while there are delalloc
> blocks still outstanding on the inode.
> 
> If this count is non-zero when ->release is next called, then do no
> truncate away the speculative preallocation - leave it there so that
> subsequent writes do not need to reallocate the delalloc space. This
> will prevent interleaving of extents of different inodes written
> concurrently to the same AG.
> 
> If we get this wrong, it is not a big deal as we truncate
> speculative allocation beyond EOF anyway in xfs_inactive() when the
> inode is thrown out of the cache.

Looks good.

> The new counter in the struct xfs_inode fits into a hole in the
> structure on 64 bit machines, so does not grow the size of the inode
> at all.


There's no counter any more. (the text further above could also use some
minor updates for that)

Reviewed-by: Christoph Hellwig <hch at lst.de>