[PATCH v4 5/6] xfs: add quota-driven speculative preallocation throttling

[Mark Tinguely]

On 02/20/13 09:10, Brian Foster wrote:
> Introduce the need_throttle() and calc_throttle() functions to
> independently check whether throttling is required for a particular
> dquot and if so, calculate the associated throttling metrics based
> on the state of the quota. We use the same general algorithm to
> calculate the throttle shift as for global free space with the
> exception of using three stages rather than five.
>
> Update xfs_iomap_prealloc_size() to use the smallest available
> prealloc size based on each of the constraints and apply the
> maximum shift to obtain the throttled preallocation size.
>
> Signed-off-by: Brian Foster<bfoster at redhat.com>
> ---

>   	/*
>   	 * MAXEXTLEN is not a power of two value but we round the prealloc down
> @@ -412,6 +472,28 @@ xfs_iomap_prealloc_size(
>   		if (freesp<  mp->m_low_space[XFS_LOWSP_1_PCNT])
>   			shift++;
>   	}
> +
> +	/*
> +	 * Check each quota to cap the prealloc size and provide a shift
> +	 * value to throttle with.
> +	 */
> +	if (xfs_quota_need_throttle(ip, XFS_DQ_USER, alloc_blocks))
> +		xfs_quota_calc_throttle(ip, XFS_DQ_USER,&qblocks,&qshift);
> +	if (xfs_quota_need_throttle(ip, XFS_DQ_GROUP, alloc_blocks))
> +		xfs_quota_calc_throttle(ip, XFS_DQ_GROUP,&qblocks,&qshift);
> +	if (xfs_quota_need_throttle(ip, XFS_DQ_PROJ, alloc_blocks))
> +		xfs_quota_calc_throttle(ip, XFS_DQ_PROJ,&qblocks,&qshift);
> +
> +	/*
> +	 * The final prealloc size is set to the minimum of free space available
> +	 * in each of the quotas and the overall filesystem.
> +	 *
> +	 * The shift throttle value is set to the maximum value as determined by
> +	 * the global low free space values and per-quota low free space values.
> +	 */
> +	alloc_blocks = MIN(alloc_blocks, qblocks);
> +	shift = MAX(shift, qshift);
> +
>   	if (shift)
>   		alloc_blocks>>= shift;
>   	/*

All the limits are applied from previous extents, quota and then the 
code from commit 055388a3 is applied:
	if (alloc_blocks < mp->m_writeio_blocks)
		alloc_blocks = mp->m_writeio_blocks;

                        ^^^^
we may not have mp->m_writeio_blocks left in the filesytem.
Doesn't the following make more sense?:

	if (alloc_blocks < mp->m_writeio_blocks)
		alloc_blocks = 0;

Thank-you,

--Mark