[PATCH 04/27] xfs: don't use speculative prealloc for small files

[Brian Foster]

On 06/12/2013 06:22 AM, Dave Chinner wrote:
> From: Dave Chinner <dchinner at redhat.com>
> 
...
> ---
>  fs/xfs/xfs_iomap.c |   13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index 8f8aaee..14be676 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -284,6 +284,15 @@ xfs_iomap_eof_want_preallocate(
>  		return 0;
>  
>  	/*
> +	 * If the file is smaller than the minimum prealloc and we are using
> +	 * dynamic preallocation, don't do any preallocation at all as it is
> +	 * likely this is the only write to the file that is going to be done.
> +	 */
> +	if (!(mp->m_flags & XFS_MOUNT_DFLT_IOSIZE) &&
> +	    XFS_ISIZE(ip) < mp->m_writeio_blocks)
> +		return 0;
> +

I stuck this on a box, got a 64k prealloc on an 8k file and then noticed
this was comparing inode size against a block count. This should
probably be:

	XFS_ISIZE(ip) < XFS_FSB_TO_B(mp, mp->m_writeio_blocks)

Given that, we avoid preallocs entirely until the file size reaches the
minimum write size (assuming dynamic prealloc mode).

> +	/*
>  	 * If there are any real blocks past eof, then don't
>  	 * do any speculative allocation.
>  	 */
> @@ -345,6 +354,10 @@ xfs_iomap_eof_prealloc_initial_size(
>  	if (mp->m_flags & XFS_MOUNT_DFLT_IOSIZE)
>  		return 0;
>  
> +	/* If the file is small, then use the minimum prealloc */
> +	if (XFS_ISIZE(ip) < mp->m_dalign)
> +		return 0;
> +
...

... and the same fsb->b conversion applies here.

If the file is larger than the minimum write size but smaller than the
stripe unit, we prealloc by the minimum prealloc size (m_writeio_blocks)
until the file grows beyond the stripe unit. That means if the stripe
unit is smaller than m_writeio_blocks or not specified, we'd just start
right into the dynamic prealloc algorithm. Seems reasonable.

Brian