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[linux-lvm] Clustered snapshots



Hi all,
  I'm trying to use the snapshot feature in a cluster environment but
I keep getting this error:

# lvcreate --size 100M --snapshot --name snap /dev/vg00/lvol1
  Clustered snapshots are not yet supported.

Reading the documentation:

 http://www.redhat.com/docs/manuals/enterprise/RHEL-5-manual/Cluster_Logical_Volume_Manager/snapshot_command.html

"LVM snapshots are not cluster-aware, so they require exclusive access
to a volume. For information on activating logical volumes on
individual nodes in a cluster, see Section 4.8, Activating Logical
Volumes on Individual Nodes in a Cluster."

"To activate logical volumes exclusively on one node, use the lvchange
-aey command."

I followed the instructions above but I get the same result:

# lvchange -aey /dev/vg00/lvol1
# lvcreate --size 100M --snapshot --name snap /dev/vg00/lvol1
  Clustered snapshots are not yet supported.

Looking at the LVM2 code (LVM2.2.02.40, tools/lvcreate.c:640):

/* FIXME Allow exclusive activation. */
if (vg_is_clustered(vg)) {
        log_error("Clustered snapshots are not yet supported.");
        return 0;
}

looks like there is no way to activate a snapshot in a clustered
volume group. Is the documentation wrong?
Is there anyone actively working on the "exclusive activation" FIXME
that is mentioned in the comment?

As a matter of fact I've been able to create a snapshot removing the
cluster flag from the volume group:

# vgchange -cn VolGroup01
  Volume group "VolGroup01" successfully changed
# lvcreate --size 100M --snapshot --name snap /dev/vg00/lvol1
  Logical volume "snap" created

What problem can arise meanwhile the cluster flag is disabled?
Should I just avoid any change to the volume group (lvcreate,
lvextend, etc...) or it might lead to really bad behaviours in any
way?

Thanks in advance,
-- 
Federico.


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