[Linux-cluster] Snapshots with GFS1/RHEL 4 (U4)

[Oliver Olsen]

Hi,
I'm currently in the process of upgrading a RHEL 3 cluster to RHEL 4,  
and I haven't been able to figure out how to use snapshots in a proper  
way. We have other RHEL 4 systems using LVM2, and I use snapshots with  
ext3 for backup with great success on those. As the demand for uptime  
on this particular cluster is cruicial (and regular filebackup using  
tar is way too slow!) I was hoping to accomplish the same kind of  
snapshots with GFS1 and RHEL 4 as with ext3.

My test-environment consists of 3 VM's in a VMware Server 1.0.3  
environment, using DLM and one GFS filesystem on the VM's. Fencing and  
connectivity is working just fine, but when I try to use snapshots  
things go wrong.

I try to initiate the snapshot via
[root at gfs1 Scripts]# lvcreate -L500M -s -n snap /dev/GFS/LV1
Logical volume "snap" created

When I try to mount the snapshot (in the same way as I do with an ext3  
snapshot) it does not seem to work

[root at gfs1 Scripts]# mount -t gfs
/dev/mapper/GFS-LV1 on /mnt/GFS type gfs (rw)

[root at gfs1 Scripts]# mount -t gfs /dev/GFS/snap /mnt/GFS-snap
mount: File exists

Also - when I try to do changes in the /mnt/GFS filesystem after  
creating the snapshot, there does not seem to be any changes in the  
snapshot when I check the graphical interface of LVM (it says Snapshot  
usage: 0%). Attributes are "swi-a-" and GFS (clustered) according to  
LVM.

I assume I am missing something vital here, but I haven't been able to  
find the documentation which explains this in an easy manner.
Any inputs will be highly appreciated!

Best regards,
Oliver Olsen