Re: [PATCH 0/2] ceph osd: initial VMware VAAI support

[David Disseldorp <ddiss@suse.de>]

Hi Josh,

On Thu, 10 Mar 2016 14:45:38 -0800, Josh Durgin wrote:

> On 03/10/2016 04:04 AM, David Disseldorp wrote:
> > On Thu, 10 Mar 2016 00:36:55 -0600, Mike Christie wrote:
> >
> > ...
> >>> This does not include support for XCOPY/extended copy. I
> >>> am still looking into this, but it seems it might be
> >>> difficult to support due to rbd being more tuned to cloning
> >>> entire devices. When we implement VASA, the cloneVirtualVolume
> >>> might be something we can support though.
> >
> > I suppose the src-and-dest-in-same-pg requirement would complicate
> > things quite a bit, but wouldn't clonerange be an option for XCOPY
> > offloads?
> 
> It's not a good fit, since with multiple clones putting data on the
> same set of osds, the workload and space utilization gets skewed for
> that set of osds compared to the rest of the cluster.
> 
> It also won't give you fast cloning - it's a full copy on xfs, and
> you'd need to do one for every object affected.

Currently the copy is being done on the LIO iSCSI gateway, so offloading
any of that to the OSDs would save a lot of network traffic.

Also as Ric mentioned, XFS has clone-range support coming, so Ceph's
dedupe COW optimisations need not only be limited the Btrfs Filestore.

> Due to these limitations, lack of existing clonerange use, and the
> complications it brings to the osd as the only op affecting more than
> one object, we've talked about removing the clonerange op.

Okay, fair enough. Thanks for the details.

Cheers, David