On Tue, Dec 18, 2018 at 1:11 AM Hector Martin <hec...@marcansoft.com> wrote:
> Hi list, > > I'm running libvirt qemu guests on RBD, and currently taking backups by > issuing a domfsfreeze, taking a snapshot, and then issuing a domfsthaw. > This seems to be a common approach. > > This is safe, but it's impactful: the guest has frozen I/O for the > duration of the snapshot. This is usually only a few seconds. > Unfortunately, the freeze action doesn't seem to be very reliable. > Sometimes it times out, leaving the guest in a messy situation with > frozen I/O (thaw times out too when this happens, or returns success but > FSes end up frozen anyway). This is clearly a bug somewhere, but I > wonder whether the freeze is a hard requirement or not. > > Are there any atomicity guarantees for RBD snapshots taken *without* > freezing the filesystem? Obviously the filesystem will be dirty and will > require journal recovery, but that is okay; it's equivalent to a hard > shutdown/crash. But is there any chance of corruption related to the > snapshot being taken in a non-atomic fashion? RBD snapshots are indeed crash-consistent. :) -Greg Filesystems and > applications these days should have no trouble with hard shutdowns, as > long as storage writes follow ordering guarantees (no writes getting > reordered across a barrier and such). > > Put another way: do RBD snapshots have ~identical atomicity guarantees > to e.g. LVM snapshots? > > If we can get away without the freeze, honestly I'd rather go that > route. If I really need to pause I/O during the snapshot creation, I > might end up resorting to pausing the whole VM (suspend/resume), which > has higher impact but also probably a much lower chance of messing up > (or having excess latency), since it doesn't involve the guest OS or the > qemu agent at all... > > -- > Hector Martin (hec...@marcansoft.com) > Public Key: https://marcan.st/marcan.asc > _______________________________________________ > ceph-users mailing list > ceph-users@lists.ceph.com > http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com >
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