Re: [PATCH 4/4] xfs: force writes to delalloc regions to unwritten

[Dave Chinner <david@fromorbit.com>]

On Thu, May 21, 2020 at 07:53:23PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> When writing to a delalloc region in the data fork, commit the new
> allocations (of the da reservation) as unwritten so that the mappings
> are only marked written once writeback completes successfully.  This
> fixes the problem of stale data exposure if the system goes down during
> targeted writeback of a specific region of a file, as tested by
> generic/042.
> 
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: Brian Foster <bfoster@redhat.com>

Observation: yesterday I forced a 4kB file create workload
to use unwritten extents by setting an extent size hint. That caused
about 4,500 xfs-conv kworker threads to be spawned by the workload
which had 16 userspace processes creating files...

I expect that any sort of "create lots of small files" worklaod is
going to cause xfs-conv kworker explosions, so be prepared for users
to start reporting kworker explosions with this in place...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com