Re: New copyfile system call - discuss before LSF?

Ric Wheeler
2013-02-25T21:51:12Z

On 02/25/2013 04:14 PM, Andy Lutomirski wrote:
> On 02/21/2013 02:24 PM, Zach Brown wrote:
>> On Thu, Feb 21, 2013 at 08:50:27PM +0000, Myklebust, Trond wrote:
>>> On Thu, 2013-02-21 at 21:00 +0100, 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.
>>> So how would a system call based on sendfile64() plus my flag parameter
>>> prevent an underlying implementation from meeting your criterion?
>> If I'm guessing correctly, sendfile64()+flags would be annoying because
>> it's missing an out_fd_offset. The host will want to offload the
>> guest's copies by calling sendfile on block ranges of a guest disk image
>> file that correspond to the mappings of the in and out files in the
>> guest.
>>
>> You could make it work with some locking and out_fd seeking to set the
>> write offset before calling sendfile64()+flags, but ugh.
>>
>> ssize_t sendfile(int out_fd, int in_fd, off_t in_offset, off_t
>> out_offset, size_t count, int flags);
>>
>> That seems closer.
>>
>> We might also want to pre-emptively offer iovs instead of offsets,
>> because that's the very first thing that's going to be requested after
>> people prototype having to iterate calling sendfile() for each
>> contiguous copy region.
> I thought the first thing people would ask for is to atomically create a
> new file and copy the old file into it (at least on local file systems).
> The idea is that nothing should see an empty destination file, either
> by race or by crash. (This feature would perhaps be described as a
> pony, but it should be implementable.)
>
> This would be like a better link(2).
>
> --Andy

Why would this need to be atomic? That would seem to be a very difficult
property to provide across all target types with multi-GB sized files...

Ric