Re: [PATCH 2/2] xfs: relax unwritten writeback overhead under some circumstances

Christoph Hellwig
2020-01-16T16:49:04Z

On Wed, Jan 15, 2020 at 10:15:58PM -0800, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
>
> In the previous patch, we solved a stale disk contents exposure problem
> by forcing the delalloc write path to create unwritten extents, write
> the data, and convert the extents to written after writeback completes.
>
> This is a pretty huge hammer to use, so we'll relax the delalloc write
> strategy to go straight to written extents (as we once did) if someone
> tells us to write the entire file to disk. This reopens the exposure
> window slightly, but we'll only be affected if writeback completes out
> of order and the system crashes during writeback.
>
> Because once again we can map written extents past EOF, we also
> enlarge the writepages window downward if the window is beyond the
> on-disk size and there are written extents after the EOF block. This
> ensures that speculative post-EOF preallocations are not left uncovered.

This does sound really sketchy. Do you have any performance numbers
justifying something this nasty?