librbd pseudo-automatically handles this by flushing the cache to the snapshot when a new snapshot is created, but I don't think krbd does the same. If it doesn't, it would probably be a nice addition to the block driver to support the general case.
Baring that (or if you want to involve something like fsfreeze), I think the answer depends on how much you are willing to write some custom C/C++ code (I don't think the rados python library exposes watch/notify APIs). A daemon could register a watch on a custom per-host/image/etc object which would sync the disk when a notification is received. Prior to creating a snapshot, you would need to send a notification to this object to alert the daemon to sync/fsfreeze/etc.
On Thu, Jul 7, 2016 at 12:33 PM, Nick Fisk <nick@xxxxxxxxxx> wrote:
Hi All,
I have a RBD mounted to a machine via the kernel client and I wish to be able to take a snapshot and mount it to another machine
where it can be backed up.
The big issue is that I need to make sure that the process writing on the source machine is finished and the FS is sync'd before
taking the snapshot.
My question. Is there something I can do with Watch/Notify to trigger this checking/sync process on the source machine before the
snapshot is actually taken?
Thanks,
Nick
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Jason
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