On Thu, 9 Aug 2018, Piotr Dałek wrote: > Hello, > > At OVH we're heavily utilizing snapshots for our backup system. We think > there's an interesting optimization opportunity regarding snapshots I'd like > to discuss here. > > The idea is to introduce a concept of a "lightweight" snapshots - such > snapshot would not contain data but only the information about what has > changed on the image since it was created (so basically only the object map > part of snapshots). > > Our backup solution (which seems to be a pretty common practice) is as > follows: > > 1. Create snapshot of the image we want to backup > 2. If there's a previous backup snapshot, export diff and apply it on the > backup image > 3. If there's no older snapshot, just do a full backup of image > > This introduces one big issue: it enforces COW snapshot on image, meaning that > original image access latencies and consumed space increases. "Lightweight" > snapshots would remove these inefficiencies - no COW performance and storage > overhead. The snapshot in 1 would be lightweight you mean? And you'd do the backup some (short) time later based on a diff with changed extents? I'm pretty sure this will export a garbage image. I mean, it will usually be non-garbage, but the result won't be crash consistent, and in some (many?) cases won't be usable. Consider: - take reference snapshot - back up this image (assume for now it is perfect) - write A to location 1 - take lightweight snapshot - write B to location 1 - backup process copie location 1 (B) to target That's the wrong data. Maybe that change is harmless, but maybe location 1 belongs to the filesystem journal, and you have some records that now reference location 10 that as an A-era value, or haven't been written at all yet, and now your file system journal won't replay and you can't mount... sage > At first glance, it seems like it could be implemented as extension to current > RBD snapshot system, leaving out the machinery required for copy-on-write. In > theory it could even co-exist with regular snapshots. Removal of these > "lightweight" snapshots would be instant (or near instant). > > So what do others think about this? > > -- > Piotr Dałek > piotr.dalek@xxxxxxxxxxxx > https://www.ovhcloud.com > -- > 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 > >
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