linux-kernel - Re: [PATCH 0/2] null

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Date:   Tue, 10 Jul 2018 12:51:55 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Bart Van Assche <Bart.VanAssche@....com>,
        "mb@...htnvm.io" <mb@...htnvm.io>,
        "loberman@...hat.com" <loberman@...hat.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        Damien Le Moal <Damien.LeMoal@....com>
Subject: Re: [PATCH 0/2] null_blk: zone support

On 7/10/18 12:49 PM, Bart Van Assche wrote:
> On Tue, 2018-07-10 at 12:45 -0600, Jens Axboe wrote:
>> The difference between the job file and the
>> workload run can be huge. Consider something really basic:
>>
>> [randwrites]
>> bs=4k
>> rw=randwrite
>>
>> which would be 100% random 4k writes. If I run this on a zoned device,
>> then that'd turn into 100% sequential writes.
> 
> That's not correct. The ZBD code in the github pull request serializes writes
> per zone, not globally.

That's a totally minor detail. If all my random writes fall within a single
zone, then they'd be 100% sequential. For N open zones, you'd be 100%
sequential within the zone. The point is that the workload as defined and
the workload as run are two totally different things, and THAT is the
problem.

-- 
Jens Axboe