Re: [PATCH V2 10/12] scsi: sd_zbc: Disable zone write locking with scsi-mq

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Hi Damien,

On Fri, Sep 08, 2017 at 01:16:38AM +0900, Damien Le Moal wrote:
> In the case of a ZBC disk used with scsi-mq, zone write locking does
> not prevent write reordering in sequential zones. Unlike the legacy
> case, zone locking can only be done after the command request is
> removed from the scheduler dispatch queue. That is, at the time of
> zone locking, the write command may already be out of order.

Per my understanding, for legacy case, it can be quite tricky to let
the existed I/O scheduler guarantee the write order for ZBC disk.
I guess requeue still might cause write reorder even in legacy path,
since requeue can happen in both scsi_request_fn() and scsi_io_completion()
with q->queue_lock released, meantime new rq belonging to the same
zone can come and be inserted to queue.

> 
> Disable zone write locking in sd_zbc_write_lock_zone() if the disk is
> used with scsi-mq. Write order guarantees can be provided by an
> adapted I/O scheduler.

Sounds a good idea to enhance the order in a new scheduler, will
look at the following patch.

-- 
Ming



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