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From:   Damien Le Moal <Damien.LeMoal@wdc.com>
To:     Kanchan Joshi <joshiiitr@gmail.com>
CC:     Christoph Hellwig <hch@lst.de>,
        Kanchan Joshi <joshi.k@samsung.com>,
        Jens Axboe <axboe@kernel.dk>,
        "sagi@grimberg.me" <sagi@grimberg.me>,
        Johannes Thumshirn <Johannes.Thumshirn@wdc.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-nvme@lists.infradead.org" <linux-nvme@lists.infradead.org>,
        Keith Busch <kbusch@kernel.org>,
        Selvakumar S <selvakuma.s1@samsung.com>,
        Javier Gonzalez <javier.gonz@samsung.com>,
        Nitesh Shetty <nj.shetty@samsung.com>
Subject: Re: [PATCH 1/2] nvme: set io-scheduler requirement for ZNS
Thread-Topic: [PATCH 1/2] nvme: set io-scheduler requirement for ZNS
Thread-Index: AQHWdSEIMmCxi6h3l0CfpdyAr+vLMA==
Date:   Mon, 7 Sep 2020 08:22:13 +0000
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On 2020/09/07 16:01, Kanchan Joshi wrote:=0A=
>> Even for SMR, the user is free to set the elevator to none, which disabl=
es zone=0A=
>> write locking. Issuing writes correctly then becomes the responsibility =
of the=0A=
>> application. This can be useful for settings that for instance use NCQ I=
/O=0A=
>> priorities, which give better results when "none" is used.=0A=
> =0A=
> Was it not a problem that even if the application is sending writes=0A=
> correctly, scheduler may not preserve the order.=0A=
> And even when none is being used, re-queue can happen which may lead=0A=
> to different ordering.=0A=
=0A=
"Issuing writes correctly" means doing small writes, one per zone at most. =
In=0A=
that case, it does not matter if the block layer reorders writes. Per zone,=
 they=0A=
will still be sequential.=0A=
=0A=
>> As far as I know, zoned drives are always used in tightly controlled=0A=
>> environments. Problems like "does not know what other applications would=
 be=0A=
>> doing" are non-existent. Setting up the drive correctly for the use case=
 at hand=0A=
>> is a sysadmin/server setup problem, based on *the* application (singular=
)=0A=
>> requirements.=0A=
> =0A=
> Fine.=0A=
> But what about the null-block-zone which sets MQ-deadline but does not=0A=
> actually use write-lock to avoid race among multiple appends on a=0A=
> zone.=0A=
> Does that deserve a fix?=0A=
=0A=
In nullblk, commands are executed under a spinlock. So there is no concurre=
ncy=0A=
problem. The spinlock serializes the execution of all commands. null_blk zo=
ne=0A=
append emulation thus does not need to take the scheduler level zone write =
lock=0A=
like scsi does.=0A=
=0A=
=0A=
=0A=
-- =0A=
Damien Le Moal=0A=
Western Digital Research=0A=