[linux-lvm] is LVM cluster-aware?

[linux-lvm] is LVM cluster-aware?

[Tomasz Chmielewski]

Is LVM cluster-aware? Of course it's not, but... Will I loose data if I 
have a LVM physical volume replicated on two machines with DRBD, and I 
want to access two separate logical volumes on different machines?


Machine_1
          \            /--Logical_volume_1
           DRBD-LVM2-PV
          /            \--Logical_volume_2
Machine_2


On Machine_1 I only want to read/write to Logical_volume_1.
On Machine_2 I only want to read/write to Logical_volume_2.

I don't want to add/remove/rename/resize/etc. any volumes - just access 
them on separate machines.


Perhaps the answer is: yes, you will loose your data, but I wanted to be 
sure ;)

Are there any workarounds to that (with just two volumes it's simple - 
don't use LVM; but let's suppose there are many logical volumes)?


-- 
Tomasz Chmielewski
http://wpkg.org

Re: [linux-lvm] is LVM cluster-aware?

[Bryn M. Reeves]

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Tomasz Chmielewski wrote:
> Is LVM cluster-aware? Of course it's not, but... Will I loose data if I
> have a LVM physical volume replicated on two machines with DRBD, and I
> want to access two separate logical volumes on different machines?

LVM2 supports clustering with several generations of Red Hat cluster
(RHEL4 cman, RHEL5 openais with dlm or gulm locking) via the clvmd
deamon. Once a volume group is marked as clustered & the infrastructure
is set up, the tools automatically lock and unlock LVM resources across
the cluster. This prevents a tool running on one node from interfering
with another tool running at the same time on a different node.

If you need it to work with another clustering environment you'll need
to make code changes - see the code in daemons/clvmd/ for the existing
cluster types.

> I don't want to add/remove/rename/resize/etc. any volumes - just access
> them on separate machines.

As long as you never try to update the LVM metadata from two nodes at
the same time (e.g. add/remove/rename/resize etc. running on two nodes
simultaneously) then this is fine, but there is no automatic prevention
of this situation.

If/when you do need to make changes, you'll want to make sure the VG you
are working with is only active on one node.

See various threads on this mailing list & linux-cluster for further
discussion of clvmd and using LVM in a clustered environment without it.

Regards,
Bryn.

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Re: [linux-lvm] is LVM cluster-aware?

[Tomasz Chmielewski]

Bryn M. Reeves schrieb:

>> I don't want to add/remove/rename/resize/etc. any volumes - just access
>> them on separate machines.
> 
> As long as you never try to update the LVM metadata from two nodes at
> the same time (e.g. add/remove/rename/resize etc. running on two nodes
> simultaneously) then this is fine, but there is no automatic prevention
> of this situation.

Thanks for the tips on clvmd and pointing to linux-cluster list archive 
- I'm about to dig into it.

Are you 100% sure LVM metadata is not updated when I don't 
add/remove/rename/resize/etc. any volumes?

I.e., won't lvs or pvs commands write or update some timestamps, for 
example?


-- 
Tomasz Chmielewski
http://wpkg.org