linux-kernel - Re: [RFC PATCH v2 0/2] add simple copy support

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Date:   Tue, 8 Dec 2020 13:24:40 +0000
From:   Johannes Thumshirn <Johannes.Thumshirn@....com>
To:     Javier González <javier@...igon.com>
CC:     Hannes Reinecke <hare@...e.de>, Christoph Hellwig <hch@....de>,
        SelvaKumar S <selvakuma.s1@...sung.com>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "kbusch@...nel.org" <kbusch@...nel.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        Damien Le Moal <Damien.LeMoal@....com>,
        "sagi@...mberg.me" <sagi@...mberg.me>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dm-devel@...hat.com" <dm-devel@...hat.com>,
        "snitzer@...hat.com" <snitzer@...hat.com>,
        "selvajove@...il.com" <selvajove@...il.com>,
        "nj.shetty@...sung.com" <nj.shetty@...sung.com>,
        "joshi.k@...sung.com" <joshi.k@...sung.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Bart Van Assche <bvanassche@....org>,
        Mikulas Patocka <mpatocka@...hat.com>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>
Subject: Re: [RFC PATCH v2 0/2] add simple copy support

On 08/12/2020 14:13, Javier González wrote:
> On 08.12.2020 12:37, Johannes Thumshirn wrote:
>> On 08/12/2020 13:22, Javier González wrote:
>>> Good idea. Are you thinking of a sysfs entry to select the backend?
>>
>> Not sure on this one, initially I thought of a sysfs file, but then
>> how would you do it. One "global" sysfs entry is probably a bad idea.
>> Having one per block device to select native vs emulation maybe? And
>> a good way to benchmark.
> 
> I was thinking a per block device to target the use case where a certain
> implementation / workload is better one way or the other.

Yes something along those lines.

>>
>> The other idea would be a benchmark loop on boot like the raid library
>> does.
>>
>> Then on the other hand, there might be workloads that run faster with
>> the emulation and some that run faster with the hardware acceleration.
>>
>> I think these points are the reason the last attempts got stuck.
> 
> Yes. I believe that any benchmark we run would be biased in a certain
> way. If we can move forward with a sysfs entry and default to legacy
> path, we would not alter current behavior and enable NVMe copy offload
> (for now) for those that want to use it. We can then build on top of it.
> 
> Does this sound like a reasonable approach?
> 

Yes this sounds like a reasonable approach to me.