Limitation: Snapshots does not work in cluster enviroment ?
Hi all,
i read, snapshots are not supported in a cluster enviroment (CLVM)
http://sources.redhat.com/lvm2/wiki/RoadMap
So, is it not possible to create a LVM2 snapshot on one server and use it in another server ?
QUESTION: Is this limitation also valid for my verion:
xentor:/etc/lvm# lvm version LVM version: 2.02.07 (2006-07-17) Library version: 1.02.08 (2006-07-17) Driver version: 4.7.0
If so, is it valid for the current LVM2 2.02.24 ?
Thanks for helping
regards
Danny
Quoted 2 lines · Daniel Schwager
So, is it not possible to create a LVM2 snapshot on one server and use it in another server ?
Not simultaneously. Origin+Snapshot(s) can only be used on one machine at once.
Quoted 1 line
QUESTION: Is this limitation also valid for my verion:
These are limitiations of the kernel implementation, not userspace.
Alasdair
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Quoted 9 lines · Alasdair G Kergon
Quoted 3 lines · Daniel Schwager
So, is it not possible to create a LVM2 snapshot on one server and use it in another server ?
Not simultaneously. Origin+Snapshot(s) can only be used on one machine at once.
So, i want to use snapshot1 on server1, snapshot2 on server2. The original LV (where the snapshot is created from) is not in use (not mounted on any server).
Is this possible ?
Quoted 7 lines
Quoted 3 lines
QUESTION: Is this limitation also valid for my verion:
These are limitiations of the kernel implementation, not userspace.
Is there a kernel-patch available ?
regards
Danny
Quoted 6 lines · Daniel Schwager
So, i want to use snapshot1 on server1, snapshot2 on server2. The original LV
(where the snapshot is created from) is not in use (not mounted on any server).Is this possible ?
Well the reason you can't use snapshots in a cluster is because writes cannot be handled correctly when there's more than one machine involved. If all your writes are local with no impact on LVs in use on other machines then the current kernel implementation ought to work. I think the userspace LVM code could be tweaked to support this configuration. Until then, you'll need to activate the snapshots manually using dmsetup directly.
Userspace changes we'd need for the tools to support this:
Permit a snapshot to be activated without its corresponding origin.
- requires a snapshot exclusive lock (already supported)
- requires an origin lock that prevents any node having origin active
(implementation needs more thought but it should be possible)
Alasdair
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Quoted 2 lines · Alasdair G Kergon
Quoted 2 lines
So, i want to use snapshot1 on server1, snapshot2 on server2. The original LV (where the snapshot is created from) is not in use (not mounted on any server)
..
Quoted 4 lines
I think the userspace LVM code could be tweaked to support this configuration. Until then, you'll need to activate the snapshots manually using dmsetup directly.
This would be fine.
Quoted 8 lines
Userspace changes we'd need for the tools (dmsetup) to support this:
Permit a snapshot to be activated without its corresponding origin.
- requires a snapshot exclusive lock (already supported)
- requires an origin lock that prevents any node having origin active
(implementation needs more thought but it should be possible)
So, thank you very much for your detailed explanation. Would
an implementaion change on "dmsetup" also solve the "Limitation of 32 snapshots" ? Or is there a another possibility to fix/workaround this topic ?
regards
Danny
Quoted 3 lines · Daniel Schwager
So, thank you very much for your detailed explanation. Would an implementaion change on "dmsetup" also solve the "Limitation of 32 snapshots" ?
Yes - that is a in-use per-machine limitation, and you'd only be using one snapshot on each machine.
Alasdair
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Quoted 10 lines · Alasdair G Kergon
Quoted 3 lines · Daniel Schwager
So, thank you very much for your detailed explanation. Would
an implementaion change on "dmsetup" also solve the "Limitation of 32 snapshots" ?Yes - that is a in-use per-machine limitation, and you'd only be using one snapshot on each machine.
Do RH or the community want in integrate the changes in dmsetup - is there maybe a timeline for this ? My task is to take n (e.g. n=5) identical snapshot(snap1-5.LV) from a xen-image (base.LV) and run 5 Xen-VM's (based on snap1-5.LV) on 5 different XEN-Server (phy.HW).
If this could not done with CLVM2, i have to change the concept to use GFS/OCFS2.
regards
Danny