[RFC PATCH v2 0/2] add simple copy support

[javier.gonz@samsung.com]

On 07.12.2020 08:06, Damien Le Moal wrote:
>On 2020/12/07 16:46, javier.gonz at samsung.com wrote:
>> On 04.12.2020 23:40, Keith Busch wrote:
>>> On Fri, Dec 04, 2020 at 11:25:12AM +0000, Damien Le Moal wrote:
>>>> On 2020/12/04 20:02, 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
>>>>>
>>>>> This is an RFC. Looking forward for any feedbacks or other alternate
>>>>> designs for plumbing simple copy to IO stack.
>>>>>
>>>>> 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 accepts destination, no of sources and arrays of
>>>>> source ranges from application and attach it as payload to the bio and
>>>>> submits to the device.
>>>>>
>>>>> Following limits are added to queue limits and are exposed in sysfs
>>>>> to userspace
>>>>> 	- *max_copy_sectors* limits the sum of all source_range length
>>>>> 	- *max_copy_nr_ranges* limits the number of source ranges
>>>>> 	- *max_copy_range_sectors* limit the maximum number of sectors
>>>>> 		that can constitute a single source range.
>>>>
>>>> Same comment as before. I think this is a good start, but for this to be really
>>>> useful to users and kernel components alike, this really needs copy emulation
>>>> for drives that do not have a native copy feature, similarly to what write zeros
>>>> handling for instance: if the drive does not have a copy command (simple copy
>>>> for NVMe or XCOPY for scsi), then the block layer should issue read/write
>>>> commands to seamlessly execute the copy. Otherwise, this will only serve a small
>>>> niche for users and will not be optimal for FS and DM drivers that could be
>>>> simplified with a generic block layer copy functionality.
>>>>
>>>> This is my 10 cents though, others may differ about this.
>>>
>>> Yes, I agree that copy emulation support should be included with the
>>> hardware enabled solution.
>>
>> Keith, Damien,
>>
>> Can we do the block layer emulation with this patchset and then work in
>> follow-up patchses on (i) the FS interface with F2FS as a first user and
>> (ii) other HW accelerations such as XCOPY?
>
>The initial patchset supporting NVMe simple copy and emulation copy, all under
>an API that probably will be similar that of dm-kcopyd will cover all block
>devices. Other hardware native support for copy functions such as scsi extended
>copy can be added later under the hood without any API changes (or minimal changes).

Sounds good. That we can do. We will add a new patch for this.

>
>I am not sure what you mean by "FS interface for F2FS": the block layer API for
>this copy functionality will be what F2FS (and other FSes) will call. That is
>the interface, no ?

Essentially yes.. I mean adding the F2FS logic and potentially some
helpers to the block layer to aid GC.

>
>> For XCOPY, I believe we need to have a separate discussion as much works
>> is already done that we should align to.
>
>I think Martin (added to this thread) and others have looked into it but I do
>not think that anything made it into the kernel yet.

Exactly. Looking at some of the code posted through time and recalling
the discussions at LSF/MM, seems like there are a number of things we
are not addressing here that could be incorporated down the road, such
as dedicated syscalls / extensions, multi namespace / device support,
etc.
>