Re: [PATCH] xfs: defer online discard submission to a workqueue

[Christoph Hellwig <hch@infradead.org>]

On Tue, Nov 06, 2018 at 09:23:11AM -0500, Brian Foster wrote:
> My
> understanding is that these discards can stack up and take enough time
> that a limit on outstanding discards is required, which now that I think
> of it makes me somewhat skeptical of the whole serial execution thing.
> Hitting that outstanding discard request limit is what bubbles up the
> stack and affects XFS by holding up log forces, since new discard
> submissions are presumably blocked on completion of the oldest
> outstanding request.

We don't do strict ordering or request, but eventually requests
waiting for completion will block others from being submitted.

> I'm not quite sure what happens in the block layer if that limit were
> lifted. Perhaps it assumes throttling responsibility directly via
> queues/plugs? I'd guess that at minimum we'd end up blocking indirectly
> somewhere (via memory allocation pressure?) anyways, so ISTM that some
> kind of throttling is inevitable in this situation. What am I missing?

We'll still block new allocations waiting for these blocks and
other bits.  Or to put it another way - if your discard implementation
is slow (independent of synchronous or not) your are going to be in
a world of pain with online discard.  That is what it's not default
to start with.