linux-kernel - Re: [RFC PATCH v5 0/4] add simple copy support

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Message-ID: <20210413182558.v2lynge6aleazgbv@mpHalley.localdomain>
Date:   Tue, 13 Apr 2021 20:25:58 +0200
From:   Javier González <javier@...igon.com>
To:     Max Gurtovoy <mgurtovoy@...dia.com>
Cc:     Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>,
        SelvaKumar S <selvakuma.s1@...sung.com>,
        linux-nvme@...ts.infradead.org, axboe@...nel.dk,
        Damien Le Moal <damien.lemoal@....com>, kch@...nel.org,
        sagi@...mberg.me, snitzer@...hat.com, selvajove@...il.com,
        linux-kernel@...r.kernel.org, nj.shetty@...sung.com,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        dm-devel@...hat.com, joshi.k@...sung.com, kbusch@...nel.org,
        joshiiitr@...il.com, hch@....de
Subject: Re: [RFC PATCH v5 0/4] add simple copy support

On 13.04.2021 18:38, Max Gurtovoy wrote:
>
>On 4/11/2021 10:26 PM, Javier González wrote:
>>On 11.04.2021 12:10, Max Gurtovoy wrote:
>>>
>>>On 4/10/2021 9:32 AM, Javier González wrote:
>>>>>On 10 Apr 2021, at 02.30, Chaitanya Kulkarni 
>>>>><Chaitanya.Kulkarni@....com> wrote:
>>>>>
>>>>>On 4/9/21 17:22, Max Gurtovoy wrote:
>>>>>>>On 2/19/2021 2:45 PM, SelvaKumar S wrote:
>>>>>>>This patchset tries to add support for TP4065a ("Simple 
>>>>>>>Copy Command"),
>>>>>>>v2020.05.04 ("Ratified")
>>>>>>>
>>>>>>>The Specification can be found in following link.
>>>>>>>https://nvmexpress.org/wp-content/uploads/NVM-Express-1.4-Ratified-TPs-1.zip
>>>>>>>
>>>>>>>
>>>>>>>Simple copy command is a copy offloading operation and is  
>>>>>>>used to copy
>>>>>>>multiple contiguous ranges (source_ranges) of LBA's to a 
>>>>>>>single destination
>>>>>>>LBA within the device reducing traffic between host and device.
>>>>>>>
>>>>>>>This implementation doesn't add native copy offload 
>>>>>>>support for stacked
>>>>>>>devices rather copy offload is done through emulation. Possible use
>>>>>>>cases are F2FS gc and BTRFS relocation/balance.
>>>>>>>
>>>>>>>*blkdev_issue_copy* takes source bdev, no of sources, 
>>>>>>>array of source
>>>>>>>ranges (in sectors), destination bdev and destination 
>>>>>>>offset(in sectors).
>>>>>>>If both source and destination block devices are same and 
>>>>>>>copy_offload = 1,
>>>>>>>then copy is done through native copy offloading. Copy 
>>>>>>>emulation is used
>>>>>>>in other cases.
>>>>>>>
>>>>>>>As SCSI XCOPY can take two different block devices and no 
>>>>>>>of source range is
>>>>>>>equal to 1, this interface can be extended in future to 
>>>>>>>support SCSI XCOPY.
>>>>>>Any idea why this TP wasn't designed for copy offload between 2
>>>>>>different namespaces in the same controller ?
>>>>>Yes, it was the first attempt so to keep it simple.
>>>>>
>>>>>Further work is needed to add incremental TP so that we can 
>>>>>also do a copy
>>>>>between the name-spaces of same controller (if we can't 
>>>>>already) and to the
>>>>>namespaces that belongs to the different controller.
>>>>>
>>>>>>And a simple copy will be the case where the src_nsid == dst_nsid ?
>>>>>>
>>>>>>Also why there are multiple source ranges and only one dst range ? We
>>>>>>could add a bit to indicate if this range is src or dst..
>>>>One of the target use cases was ZNS in order to avoid fabric 
>>>>transfers during host GC. You can see how this plays well with 
>>>>several zone ranges and a single zone destination.
>>>>
>>>>If we start getting support in Linux through the different past 
>>>>copy offload efforts, I’m sure we can extend this TP in the 
>>>>future.
>>>
>>>But the "copy" command IMO is more general than the ZNS GC case, 
>>>that can be a private case of copy, isn't it ?
>>
>>It applies to any namespace type, so yes. I just wanted to give you the
>>background for the current "simple" scope through one of the use cases
>>that was in mind.
>>
>>>We can get a big benefit of offloading the data copy from one ns 
>>>to another in the same controller and even in different 
>>>controllers in the same subsystem.
>>
>>Definitely.
>>
>>>
>>>Do you think the extension should be to "copy" command or to 
>>>create a new command "x_copy" for copying to different destination 
>>>ns ?
>>
>>I believe there is space for extensions to simple copy. But given the
>>experience with XCOPY, I can imagine that changes will be incremental,
>>based on very specific use cases.
>>
>>I think getting support upstream and bringing deployed cases is a very
>>good start.
>
>Copying data (files) within the controller/subsystem from ns_A to ns_B 
>using NVMf will reduce network BW and memory BW in the host server.
>
>This feature is well known and the use case is well known.

Definitely.

>
>The question whether we implement it in vendor specific manner of we 
>add it to the specification.
>
>I prefer adding it to the spec :)

Agree. Let's build up on top of Simple Copy. We can talk about it
offline in the context of the NVMe TWG.