Messages in this thread | | | Date | Thu, 26 Sep 2013 17:26:39 -0400 | From | Ric Wheeler <> | Subject | Re: [RFC] extending splice for copy offloading |
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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
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