Re: Reordering of ublk IO requests

[Andreas Hindborg <andreas.hindborg@wdc.com>]

Andreas Hindborg <andreas.hindborg@wdc.com> writes:

> Damien Le Moal <damien.lemoal@opensource.wdc.com> writes:
>
>> On 11/18/22 21:47, Ming Lei wrote:
>>> Anytime, there is at most one write IO for each zone, how can the single
>>> write IO be re-order?
>>
>> If the user issues writes one at a time out of order (not aligned to the
>> write pointer), mq-deadline will not help at all. The zone write locking
>> will still limit write dispatching to one per zone, but the writes will fail.
>>
>> mq-deadline will reorder write commands in the correct lba order only if:
>> - the commands are inserted as a batch (more than on request passed to
>> ->insert_requests)
>> - commands are inserted individually when the target zone is locked (a
>> write is already being executed)
>>
>> This has been the semantic from the start: the block layer has no
>> guarantees about the correct ordering of writes to zoned drive. What is
>> guaranteed is that (1) if the user issues writes in order AND (2)
>> mq-deadline is used, then writes will be dispatched in the same order to
>> the device.
>>
>> I have not looked at the details of ublk, but from the thread, I think (1)
>> is not done and (2) is missing-ish as the ublk device is not marked as zoned.
>
> I have a patch in the works for adding zoned storage support to ublk. It
> sets up the ublk device as a zoned device. It is very much work in
> progress, but it lives here [1] for now.
>
> I am pretty sure that I saw large writes to zoned ublk device being
> split and issued to the device (same zone) with multiple outstanding
> requests at the same time. I'll verify on Monday and provide a test case
> if that is the case. Might be I configured the ublk device wrong? I set
> it up as host managed zoned and set up zone size, max active, max open.

Turns out I was not setting up the zone bitmaps. I was missing a call to
blk_revalidate_disk_zones() and that took out the zone lock in the
deadline scheduler. With that in place, everything works as it should.

Thanks for the help everyone!

Best regards,
Andreas