Re: [PATCH v2] block/file-posix: optimize append write

[Kevin Wolf <kwolf@redhat.com>]

Am 20.10.2024 um 03:03 hat Damien Le Moal geschrieben:
> On 10/18/24 23:37, Kevin Wolf wrote:
> > Am 04.10.2024 um 12:41 hat Sam Li geschrieben:
> >> When the file-posix driver emulates append write, it holds the lock
> >> whenever accessing wp, which limits the IO queue depth to one.
> >>
> >> The write IO flow can be optimized to allow concurrent writes. The lock
> >> is held in two cases:
> >> 1. Assumed that the write IO succeeds, update the wp before issuing the
> >> write.
> >> 2. If the write IO fails, report that zone and use the reported value
> >> as the current wp.
> > 
> > What happens with the concurrent writes that started later and may not
> > have completed yet? Can we really just reset to the reported value
> > before all other requests have completed, too?
> 
> Yes, because if one write fails, we know that the following writes
> will fail too as they will not be aligned to the write pointer. These
> subsequent failed writes will again trigger the report zones and
> update, but that is fine. All of them have failed and the report will
> give the same wp again.
> 
> This is a typical pattern with zoned block device: if one write fails
> in a zone, the user has to expect failures for all other writes issued
> to the same zone, do a report zone to get the wp and restart writing
> from there.

Ok, that makes sense. Can we be sure that requests are handled in the
order they were submitted, though? That is, if the failed request is
resubmitted, could the already pending next one still succeed if it's
overtaken by the resubmitted request? Not sure if this would even cause
a probem, but is it a case we have to consider?

Kevin