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Subject: Re: [RFC PATCH v2 0/2] add simple copy support
To:     Christoph Hellwig <hch@lst.de>,
        SelvaKumar S <selvakuma.s1@samsung.com>
Cc:     linux-nvme@lists.infradead.org, kbusch@kernel.org, axboe@kernel.dk,
        damien.lemoal@wdc.com, sagi@grimberg.me,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        dm-devel@redhat.com, snitzer@redhat.com, selvajove@gmail.com,
        nj.shetty@samsung.com, joshi.k@samsung.com,
        javier.gonz@samsung.com,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Bart Van Assche <bvanassche@acm.org>,
        Mikulas Patocka <mpatocka@redhat.com>,
        linux-scsi@vger.kernel.org
References: <CGME20201204094719epcas5p23b3c41223897de3840f92ae3c229cda5@epcas5p2.samsung.com>
 <20201204094659.12732-1-selvakuma.s1@samsung.com>
 <20201207141123.GC31159@lst.de>
From:   Hannes Reinecke <hare@suse.de>
Message-ID: <01fe46ac-16a5-d4db-f23d-07a03d3935f3@suse.de>
Date:   Mon, 7 Dec 2020 15:56:15 +0100
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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.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
hare@suse.de                              +49 911 74053 688
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