Re: [PATCH] the dm-loop target

Christoph Hellwig
2025-03-12T06:24:08Z

On Wed, Mar 12, 2025 at 01:34:02PM +1100, Dave Chinner wrote:
> Wrong.
>
> Speculative non-blocking IO like NOWAIT is the wrong optimisation to
> make for workloads that are very likely to block in the IO path. It
> just adds overhead without adding any improvement in performance.

Note that I suspect that most or at least many loop workloads are
read-heavy. And at least for reads NOWAIT makes perfect sense.

> Getting rid of the serialised IO submission problems that the loop
> device current has will benefit *all* workloads that use the loop
> device, not just those that are fully allocated. Yes, it won't quite
> show the same performance as NOWAIT in that case, but it still
> should give 90-95% of native performance for the static file case.
> And it should also improve all the other cases, too, because now
> they will only serialise when the backing file needs IO operations to
> serialise (i.e. during allocation).

And I agree that this should be a first step.

> *cough*

The whole ublk-zoned is a bit of a bullshit thing where Ming wrote
up something that barely works to block inclusion of the zloop driver
we really need for zoned xfs testing. Please don't take it serious.