Re: New copyfile system call - discuss before LSF?

Ric Wheeler
2013-02-22T08:48:16Z

On 02/21/2013 11:13 PM, Myklebust, Trond wrote:
> On Thu, 2013-02-21 at 23:05 +0100, Ric Wheeler wrote:
>> On 02/21/2013 09:00 PM, Paolo Bonzini wrote:
>>> Il 21/02/2013 15:57, Ric Wheeler ha scritto:
>>>>> sendfile64() pretty much already has the right arguments for a
>>>>> "copyfile", however it would be nice to add a 'flags' parameter: the
>>>>> NFSv4.2 version would use that to specify whether or not to copy file
>>>>> metadata.
>>>> That would seem to be enough to me and has the advantage that it is an
>>>> relatively obvious extension to something that is at least not totally
>>>> unknown to developers.
>>>>
>>>> Do we need more than that for non-NFS paths I wonder? What does reflink
>>>> need or the SCSI mechanism?
>>> For virt we would like to be able to specify arbitrary block ranges.
>>> Copying an entire file helps some copy operations like storage
>>> migration. However, it is not enough to convert the guest's offloaded
>>> copies to host-side offloaded copies.
>>>
>>> Paolo
>> I don't think that the NFS protocol allows arbitrary ranges, but the SCSI
>> commands are ranged based.
>>
>> If I remember what the windows people said at a SNIA event a few years back,
>> they have a requirement that the target file be pre-allocated (at least for the
>> SCSI based copy). Not clear to me where they iterate over that target file to do
>> the block range copies, but I suspect it is in their kernel.
> The NFSv4.2 copy offload protocol _does_ allow the copying of arbitrary
> byte ranges. The main target for that functionality is indeed
> virtualisation and thin provisioning of virtual machines.
>

For background, here is a pointer to Fred Knight's SNIA talk on the SCSI support
for offload:

https://snia.org/sites/default/files2/SDC2011/presentations/monday/FrederickKnight_Storage_Data_Movement_Offload.pdf

and a talk from Spencer Shepler that gives some detail on the NFS spec,
including the "server side copy" bits:

https://snia.org/sites/default/files2/SDC2011/presentations/wednesday/SpencerShepler_IETF_NFSv4_Working_Group_v4.pdf

The talks both have references to the actual specs for the gory details.

Ric