[RFC PATCH v5 0/4] add simple copy support

[Javier González]

> On 10 Apr 2021, at 02.30, Chaitanya Kulkarni <Chaitanya.Kulkarni at wdc.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.