[linux-lvm] Limitation: Snapshots does not work in cluster enviroment ?

[linux-lvm] Limitation: Snapshots does not work in cluster enviroment ?

[Daniel Schwager]

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

Re: [linux-lvm] Limitation: Snapshots does not work in cluster enviroment ?

[Alasdair G Kergon]

On Sat, Apr 14, 2007 at 03:32:19PM +0200, Daniel Schwager wrote:
> 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.

> QUESTION: Is this limitation also valid for my verion:
 
These are limitiations of the kernel implementation, not userspace.

Alasdair
-- 
agk@redhat.com

Re: [linux-lvm] Limitation: Snapshots does not work in cluster enviroment ?

[Daniel Schwager]

Alasdair G Kergon schrieb:
> On Sat, Apr 14, 2007 at 03:32:19PM +0200, Daniel Schwager wrote:
>   
>> 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 ?
>   
>> 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

Re: [linux-lvm] Limitation: Snapshots does not work in cluster enviroment ?

[Alasdair G Kergon]

On Sat, Apr 14, 2007 at 04:37:51PM +0200, Daniel Schwager wrote:
> 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
-- 
agk@redhat.com

Re: [linux-lvm] Limitation: Snapshots does not work in cluster enviroment ?

[Daniel Schwager]

Alasdair G Kergon schrieb:
>> 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)
>>     
..
> 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.
>
> 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

Re: [linux-lvm] Limitation: Snapshots does not work in cluster enviroment ?

[Alasdair G Kergon]

On Sat, Apr 14, 2007 at 05:44:49PM +0200, Daniel Schwager wrote:
> 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
-- 
agk@redhat.com

Re: [linux-lvm] Limitation: Snapshots does not work in cluster enviroment ?

[Daniel Schwager]

Alasdair G Kergon schrieb:
> On Sat, Apr 14, 2007 at 05:44:49PM +0200, Daniel Schwager wrote:
>   
>> 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