Re: [PATCH 1/2] nvme: set io-scheduler requirement for ZNS

[Kanchan Joshi]

On Mon, Sep 7, 2020 at 5:07 PM Damien Le Moal <Damien.LeMoal@xxxxxxx> wrote:
>
> On 2020/09/07 20:24, Kanchan Joshi wrote:
> > On Mon, Sep 7, 2020 at 1:52 PM Damien Le Moal <Damien.LeMoal@xxxxxxx> wrote:
> >>
> >> On 2020/09/07 16:01, Kanchan Joshi wrote:
> >>>> Even for SMR, the user is free to set the elevator to none, which disables zone
> >>>> write locking. Issuing writes correctly then becomes the responsibility of the
> >>>> application. This can be useful for settings that for instance use NCQ I/O
> >>>> priorities, which give better results when "none" is used.
> >>>
> >>> Was it not a problem that even if the application is sending writes
> >>> correctly, scheduler may not preserve the order.
> >>> And even when none is being used, re-queue can happen which may lead
> >>> to different ordering.
> >>
> >> "Issuing writes correctly" means doing small writes, one per zone at most. In
> >> that case, it does not matter if the block layer reorders writes. Per zone, they
> >> will still be sequential.
> >>
> >>>> As far as I know, zoned drives are always used in tightly controlled
> >>>> environments. Problems like "does not know what other applications would be
> >>>> doing" are non-existent. Setting up the drive correctly for the use case at hand
> >>>> is a sysadmin/server setup problem, based on *the* application (singular)
> >>>> requirements.
> >>>
> >>> Fine.
> >>> But what about the null-block-zone which sets MQ-deadline but does not
> >>> actually use write-lock to avoid race among multiple appends on a
> >>> zone.
> >>> Does that deserve a fix?
> >>
> >> In nullblk, commands are executed under a spinlock. So there is no concurrency
> >> problem. The spinlock serializes the execution of all commands. null_blk zone
> >> append emulation thus does not need to take the scheduler level zone write lock
> >> like scsi does.
> >
> > I do not see spinlock for that. There is one "nullb->lock", but its
> > scope is limited to memory-backed handling.
> > For concurrent zone-appends on a zone, multiple threads may set the
> > "same" write-pointer into incoming request(s).
> > Are you referring to any other spinlock that can avoid "same wp being
> > returned to multiple threads".
>
> Checking again, it looks like you are correct. nullb->lock is indeed only used
> for processing read/write with memory backing turned on.
> We either need to extend that spinlock use, or add one to protect the zone array
> when doing zoned commands and checks of read/write against a zone wp.
> Care to send a patch ? I can send one too.

Sure, I can send.
Do you think it is not OK to use zone write-lock (same like SCSI
emulation) instead of introducing a new spinlock?


-- 
Kanchan