RE: copy offload support in Linux - new system call needed?

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On Thu, 2011-12-15 at 11:40 -0500, Loke, Chetan wrote: 
> > >
> > > Why not support something like the async-iocb?
> > 
> > You could, but that would tie copyfile() to the aio interface which was
> > one of the things that I believe Al was opposed to when we discussed
> > this at LSF/MM-2010.
> > 
> 
> virtualization vendors who support this offload do it at a layer above the guest-OS(Intra-LUN(tm) locking or whatever fancy locking). So I think 'copyfile' is going to be appealing to application-developers more than the hypervisor-vendors.

The application is thin provisioning, not the 'cp' command. When
virtualisation vendors do support this, it will mainly be as part of
their image management toolkits, not the hypervisor.

> So let's think about it from end-users perspective:
> Won't everyone replicate code to check - 'Am I done'? It will just make application folks write more (ugly)code. Because you would then have to maintain another queue/etc to check for this operation.

'Am I done' is easy: copyfile() returns with the number of bytes that
have been copied.

'Is my copyfile() syscall making progress' is the question that needs
answering.

> We can just support full-copy. Partial copies can be returned as failure.

Then you have to check the entire range on error instead of just
resuming the copy from where it stopped.

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@xxxxxxxxxx
www.netapp.com

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