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

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Message-ID: <CY4PR04MB375148979AA403CF47EADE0BE75D0@CY4PR04MB3751.namprd04.prod.outlook.com>
Date:   Wed, 19 Aug 2020 09:38:36 +0000
From:   Damien Le Moal <Damien.LeMoal@....com>
To:     Kanchan Joshi <joshiiitr@...il.com>, Christoph Hellwig <hch@....de>
CC:     Kanchan Joshi <joshi.k@...sung.com>, Jens Axboe <axboe@...nel.dk>,
        "sagi@...mberg.me" <sagi@...mberg.me>,
        Johannes Thumshirn <Johannes.Thumshirn@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        Keith Busch <kbusch@...nel.org>
Subject: Re: [PATCH 1/2] nvme: set io-scheduler requirement for ZNS

On 2020/08/19 18:27, Kanchan Joshi wrote:
> On Tue, Aug 18, 2020 at 12:46 PM Christoph Hellwig <hch@....de> wrote:
>>
>> On Tue, Aug 18, 2020 at 10:59:35AM +0530, Kanchan Joshi wrote:
>>> Set elevator feature ELEVATOR_F_ZBD_SEQ_WRITE required for ZNS.
>>
>> No, it is not.
> 
> Are you saying MQ-Deadline (write-lock) is not needed for writes on ZNS?
> I see that null-block zoned and SCSI-ZBC both set this requirement. I
> wonder how it became different for NVMe.

It is not required for an NVMe ZNS drive that has zone append native support.
zonefs and upcoming btrfs do not use regular writes, removing the requirement
for zone write locking.

In the context of your patch series, ELEVATOR_F_ZBD_SEQ_WRITE should be set only
and only if the drive does not have native zone append support. And even in that
case, since for an emulated zone append the zone write lock is taken and
released by the emulation driver itself, ELEVATOR_F_ZBD_SEQ_WRITE is required
only if the user will also be issuing regular writes at high QD. And that is
trivially controllable by the user by simply setting the drive elevator to
mq-deadline. Conclusion: setting ELEVATOR_F_ZBD_SEQ_WRITE is not needed.

-- 
Damien Le Moal
Western Digital Research