Re: [RFC] extending splice for copy offloading

Ric Wheeler
2013-09-26T21:27:57Z

On 09/26/2013 02:55 PM, Zach Brown wrote:
> On Thu, Sep 26, 2013 at 10:58:05AM +0200, Miklos Szeredi wrote:
>> On Wed, Sep 25, 2013 at 11:07 PM, Zach Brown <zab@redhat.com> wrote:
>>>> A client-side copy will be slower, but I guess it does have the
>>>> advantage that the application can track progress to some degree, and
>>>> abort it fairly quickly without leaving the file in a totally undefined
>>>> state--and both might be useful if the copy's not a simple constant-time
>>>> operation.
>>> I suppose, but can't the app achieve a nice middle ground by copying the
>>> file in smaller syscalls? Avoid bulk data motion back to the client,
>>> but still get notification every, I dunno, few hundred meg?
>> Yes. And if "cp" could just be switched from a read+write syscall
>> pair to a single splice syscall using the same buffer size. And then
>> the user would only notice that things got faster in case of server
>> side copy. No problems with long blocking times (at least not much
>> worse than it was).
> Hmm, yes, that would be a nice outcome.
>
>> However "cp" doesn't do reflinking by default, it has a switch for
>> that. If we just want "cp" and the like to use splice without fearing
>> side effects then by default we should try to be as close to
>> read+write behavior as possible. No?
> I guess? I don't find requiring --reflink hugely compelling. But there
> it is.
>
>> That's what I'm really
>> worrying about when you want to wire up splice to reflink by default.
>> I do think there should be a flag for that. And if on the block level
>> some magic happens, so be it. It's not the fs deverloper's worry any
>> more ;)
> Sure. So we'd have:
>
> - no flag default that forbids knowingly copying with shared references
> so that it will be used by default by people who feel strongly about
> their assumptions about independent write durability.
>
> - a flag that allows shared references for people who would otherwise
> use the file system shared reference ioctls (ocfs2 reflink, btrfs
> clone) but would like it to also do server-side read/write copies
> over nfs without additional intervention.
>
> - a flag that requires shared references for callers who don't want
> giant copies to take forever if they aren't instant. (The qemu guys
> asked for this at Plumbers.)
>
> I think I can live with that.
>
> - z

This last flag should not prevent a remote target device (NFS or SCSI array)
copy from working though since they often do reflink like operations inside of
the remote target device....

ric