On 12/21/2015 11:51 PM, 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 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... > I gave it a shot. callback functions are a bit new to me, but I gave it a try: https://github.com/wido/libvirt/commit/756dca8023027616f53c39fa73c52a6d8f86a223 Could you take a look? > Josh > -- > To unsubscribe from this list: send the line "unsubscribe ceph-devel" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- Wido den Hollander 42on B.V. Ceph trainer and consultant Phone: +31 (0)20 700 9902 Skype: contact42on -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
