Re: [PATCH v16 00/26] Improve write performance for zoned UFS devices

[Damien Le Moal <dlemoal@kernel.org>]

On 11/20/24 04:08, Bart Van Assche wrote:
> On 11/19/24 12:01 AM, Damien Le Moal wrote:
>> Impressive improvements but the changes are rather invasive. Have you tried
>> simpler solution like forcing unplugging a zone write plug from the driver once
>> a command is passed to the driver and the driver did not reject it ? It seems
>> like this would make everything simpler on the block layer side. But I am not
>> sure if the performance gains would be the same.
> 
> Hi Damien,
> 
> I'm not sure that the approach of submitting a new zoned write if the
> driver did not reject the previous write would result in a simpler
> solution. SCSI devices are allowed to respond to any command with a unit
> attention instead of processing the command. If a unit attention is
> reported, the SCSI core requeues the command. In other words, even with
> this approach, proper support for requeued zoned writes in the block
> layer is required.

Yes, but it would be vastly simpler because you would be guaranteed to having
only a single write request per zone being in-flight between the write plug and
the device at any time. So the requeue would not need reordering, and likely not
need any special code at all. nless I am missing something, this would be
simpler, no ?

The main question though with such approach is: does it give you the same
performance improvements as your current (more invasive) approach ?

> Additionally, that approach is not compatible with using .queue_rqs().
> While the SCSI core does not yet support a .queue_rqs() callback, I
> think it would be good to have this support in the SCSI core.

I do not understand why it is not compatible. What is the problem ?

> If we need requeuing support anyway, why to select an approach that
> probably will result in lower performance than what has been implemented
> in this patch series?

I am only trying to see if there is not a simpler approach than what you did.
The less changes, the better, right ?

-- 
Damien Le Moal
Western Digital Research