LVM + XFS + external log + snapshots

[Dave Chinner]

On Sun, Jun 23, 2013 at 06:24:39PM -0500, Eric Sandeen wrote:
> On 6/23/13 5:55 PM, Dave Chinner wrote:
> > On Fri, Jun 21, 2013 at 10:30:31PM -0700, aurfalien wrote:
> >> Hi all,
> >>
> >> So I have an XFS file system within LVM  which has an external log.
> >>
> >> My mount option in FSTAB is;
> >>
> >> /dev/vg_spock_data/lv_data	/data	xfs	logdev=/dev/sdc1,nobarrier,logbufs=8,noatime,nodiratime	1 1
> >>
> >> All is well no issues and very fast.
> >>
> >> Now I'd like to snapshot this bad boy and then run rsnapshot to create a few days backup.
> > 
> > You need to snapshot the log device as well.
> > 
> > But that is problematic in that you need to snapshot it at the same
> > time you snapshot the data volume. Hence yo'd have to do:
> > 
> > # xfs_freeze -f <filesystem>
> > # <snapshot data volume>
> > # <snapshot log volume>
> > # xfs_freeze -u <filesystem>
> > 
> > And now you can mount the snapshot with:
> > 
> > # mount /dev/vg_spock_data/datasnapshot /snapshot -o nouuid,ro,logdev=/dev/vg_spock_log/logsnapshot
> > 
> > If you can't snapshot the log device, then you can't snapshot the
> > filesystem. Yet another reason for using internal logs...
> 
> Hm, given that freezing the fs makes the log (almost) completely clean, I wonder
> if he could mount with /dev/zero (or a loopback 0-filled file) to get at the snapshot,
> especially since it's being mounted RO.
> 
> Should be safe & consistent, no?

In theory. Never tried it myself.

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com