[DRBD-user] configuring 2 services on 2 hosts

Lars Ellenberg lars.ellenberg at linbit.com
Mon Jan 10 22:12:41 CET 2011

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

On Mon, Jan 10, 2011 at 12:54:17PM -0600, J. Ryan Earl wrote:
> On Mon, Jan 10, 2011 at 12:51 PM, J. Ryan Earl <oss at jryanearl.us> wrote:
> > On Thu, Jan 6, 2011 at 1:23 PM, J <j at koarcg.com> wrote:
> >
> >> In a related question. Is it possible to take a snapshot of the secondary
> >> volume while the drbd is active? I can't find the exact link, but the user's
> >> guide mentioned not accessing the secondary AT ALL while the primary was
> >> active. I assumed that would include taking snapshots of the secondary?
> >
> >
> > A DRBD device in secondary mode will not be accessible in anyway.   IIRC,
> > the /dev/drbdX device won't even exist at this point.  If you're using
> > protocol C and have the DRBD resource on top of LV, you can still snapshot
> > it and the snapshot should be consistent for a point-in-time.
> >
> > -JR
> >
> I should say, you can still snapshot it on [the secondary node at the
> time]...
> Otherwise, if you're running active-passive with LVM on-top of DRBD, then
> the snapshot can only be instigated by the primary and the snapshot CoW
> volume will be replicated to the secondary.  Keep in mind, DRBD +
> Snapshotting will impose triple overhead on writes.

If your DRBD lives on top of a LV, you can of course snapshot
the LV _below_ drbd (on any node, does not make much difference).

Compared to snapshots with the file system directly on the LV,
there is an important difference: in that case the file system is
implicitly notified to "freeze", immediately before the snapshot is
taken, and then "thawed" again after the snapshot stuff has been setup.

With DRBD between LV and Filesystem, the filesystem is no longer
notified implicitly, and even though you don't have to, you may want to
do that explicitly (e.g. using xfs_freeze -f, -u), which of course
needs to be done on the Primary (where it is mounted).

If we are not talking about Filesystems, but anything else (VM images,
whatever), you just take your snapshot (of the lower level LC) as usual.

: Lars Ellenberg
: LINBIT | Your Way to High Availability
: DRBD/HA support and consulting http://www.linbit.com

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