On 12/21/2015 11:06 AM, Wido den Hollander wrote:
Hi,
While implementing the buildvolfrom method in libvirt for RBD I'm stuck
at some point.
$ virsh vol-clone --pool myrbdpool image1 image2
This would clone image1 to a new RBD image called 'image2'.
The code I've written now does:
1. Create a snapshot called image1@libvirt-<epochtimestamp>
2. Protect the snapshot
3. Clone the snapshot to 'image1'
wido@wido-desktop:~/repos/libvirt$ ./tools/virsh vol-clone --pool
rbdpool image1 image2
Vol image2 cloned from image1
wido@wido-desktop:~/repos/libvirt$
root@alpha:~# rbd -p libvirt info image2
rbd image 'image2':
size 10240 MB in 2560 objects
order 22 (4096 kB objects)
block_name_prefix: rbd_data.1976451ead36b
format: 2
features: layering, striping
flags:
parent: libvirt/image1@libvirt-1450724650
overlap: 10240 MB
stripe unit: 4096 kB
stripe count: 1
root@alpha:~#
But this could potentially lead to a lot of snapshots with children on
'image1'.
image1 itself will probably never change, but I'm wondering about the
negative performance impact this might have on a OSD.
Creating them isn't so bad, more snapshots that don't change don't have
much affect on the osds. Deleting them is what's expensive, since the
osds need to scan the objects to see which ones are part of the
snapshot and can be deleted. If you have too many snapshots created and
deleted, it can affect cluster load, so I'd rather avoid always
creating a snapshot.
I'd rather not hardcode a snapshot name like 'libvirt-parent-snapshot'
into libvirt. There is however no way to pass something like a snapshot
name in libvirt when cloning.
Any bright suggestions? Or is it fine to create so many snapshots?
You could have canonical names for the libvirt snapshots like you
suggest, 'libvirt-<timestamp>', and check via rbd_diff_iterate2()
whether the parent image changed since the last snapshot. That's a bit
slower than plain cloning, but with object map + fast diff it's fast
again, since it doesn't need to scan all the objects anymore.
I think libvirt would need to expand its api a bit to be able to really
use it effectively to manage rbd. Hiding the snapshots becomes
cumbersome if the application wants to use them too. If libvirt's
current model of clones lets parents be deleted before children,
that may be a hassle to hide too...
Josh
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