On Fri, Jul 8, 2016 at 5:02 AM, Jason Dillaman <jdill...@redhat.com> wrote:
> 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.
If there is a filesystem on top of /dev/rbdX, which isn't suspended,
how would krbd driver flushing the page cache help? In order for the
block device level snapshot to be consistent, the filesystem needs to
be quiesced - fsfreeze or something resembling it is the only answer
here.
Thanks,
Ilya
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