On 21-12-15 23:51, Josh Durgin wrote: > 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'll give that a try, seems like a good suggestion! I'll have to use rbd_diff_iterate() through since iterate2() is post-hammer and that will not be available on all systems. > 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... > Yes, I would love to see: - vol-snap-list - vol-snap-create - vol-snap-delete - vol-snap-revert And then: - vol-clone --snapshot <mysnap> --pool <mypool> image1 image2 But this would need some more work inside libvirt. Would be very nice though. At CloudStack we want to do as much as possible using libvirt, the more features it has there, the less we have to do in Java code :) Wido > Josh > -- > To unsubscribe from this list: send the line "unsubscribe ceph-devel" in > the body of a message to majord...@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
