Re: [PATCH] the dm-loop target

Christoph Hellwig
2025-03-18T07:57:18Z

On Tue, Mar 18, 2025 at 03:27:48PM +1100, Dave Chinner wrote:
> Yes, NOWAIT may then add an incremental performance improvement on
> top for optimal layout cases, but I'm still not yet convinced that
> it is a generally applicable loop device optimisation that everyone
> wants to always enable due to the potential for 100% NOWAIT
> submission failure on any given loop device.....

Yes, I think this is a really good first step:

1) switch loop to use a per-command work_item unconditionally, which also
has the nice effect that it cleans up the horrible mess of the
per-blkcg workers. (note that this is what the nvmet file backend has
always done with good result)
2) look into NOWAIT submission, especially for reads this should be
a clear winner and probaby done unconditionally. For writes it
might be a bit of a tradeoff if we expect the writes to allocate
a lot, so we might want some kind of tunable for it.