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

[Kanchan Joshi]

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".

-- 
Kanchan