On Mon, Oct 21, 2024 at 09:32:50PM +0900, Damien Le Moal wrote: > On 10/21/24 20:08, Kevin Wolf wrote: > > 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? > > A zoned device will always handle writes in the order they were submitted (per > zone) and that is true for emulated devices as well as real ones.
Is there serialization code in the kernel so that zoned devices behind multi-path keep requests ordered? Normally I don't assume any ordering between concurrent requests to a block device, so I'm surprised that it's safe to submit multiple writes. Stefan
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