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

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Date:   Mon, 7 Dec 2020 20:24:53 +0100
From:   "Javier González" <javier@...igon.com>
To:     Hannes Reinecke <hare@...e.de>
Cc:     Christoph Hellwig <hch@....de>,
        SelvaKumar S <selvakuma.s1@...sung.com>,
        linux-nvme@...ts.infradead.org, kbusch@...nel.org, axboe@...nel.dk,
        damien.lemoal@....com, sagi@...mberg.me,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        dm-devel@...hat.com, snitzer@...hat.com, selvajove@...il.com,
        nj.shetty@...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
Subject: Re: [RFC PATCH v2 0/2] add simple copy support

On 07.12.2020 15:56, Hannes Reinecke wrote:
>On 12/7/20 3:11 PM, Christoph Hellwig wrote:
>>So, I'm really worried about:
>>
>>  a) a good use case.  GC in f2fs or btrfs seem like good use cases, as
>>     does accelating dm-kcopyd.  I agree with Damien that lifting dm-kcopyd
>>     to common code would also be really nice.  I'm not 100% sure it should
>>     be a requirement, but it sure would be nice to have
>>     I don't think just adding an ioctl is enough of a use case for complex
>>     kernel infrastructure.
>>  b) We had a bunch of different attempts at SCSI XCOPY support form IIRC
>>     Martin, Bart and Mikulas.  I think we need to pull them into this
>>     discussion, and make sure whatever we do covers the SCSI needs.
>>
>And we shouldn't forget that the main issue which killed all previous 
>implementations was a missing QoS guarantee.
>It's nice to have simply copy, but if the implementation is _slower_ 
>than doing it by hand from the OS there is very little point in even 
>attempting to do so.
>I can't see any provisions for that in the TPAR, leading me to the 
>assumption that NVMe simple copy will suffer from the same issue.
>
>So if we can't address this I guess this attempt will fail, too.

Good point. We can share some performance data on how Simple Copy scales
in terms of bw / latency and the CPU usage. Do you have anything else in
mind?