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RE: [linux-lvm] Docs on clustered option of vgcreate



Excellent, thanks for the answer, that makes it a lot clearer.

 

So coupled with exclusion lists etc.. I could have multiple machines connect to one LUN and each have its own logical volume. Is there a concept of a “master” in the cluster or does it work in the same way as tcp with an exponential backoff when the metadata is locked by a host for changing. I am assuming that cluster aware locking would allow server-a to create a new lv but not simalteneosly allowing server-b to create a new lv too.

 

I know should be experimenting with this but will ask in the mean time while I wait for my “test lun”.

 

If host A creates a lv on a shared storage will it automatically be marked as only belonging to host A or is that something you have to do by hand. I read somewhere in the man pages about tags that you need to add to achieve such things.

 

Regards

 


From: linux-lvm-bounces redhat com [mailto:linux-lvm-bounces redhat com] On Behalf Of Jonathan Brassow
Sent: 07 February 2008 17:00
To: LVM general discussion and development
Subject: Re: [linux-lvm] Docs on clustered option of vgcreate

 

Clustering LVM (CLVM) provides a way to manage logical volumes in a consistent way.

 

By analogy, if you connect the same disk to two machines, put a file system on it (like ext3), and mount the file system on both machines, you can expect corruption.  Both machines will be attempting to create/modify/read metadata without the knowledge that someone else is doing the same...  You need a cluster aware file system in this type of scenario - like GFS.  The same thing is true of LVM... you don't want to have two machines sharing the same volume group - extending/shrinking/creating/deleting logical volumes without knowledge of the other machine... it would lead to corruption of your volume group layout.  This is what CLVM is all about.  It is especially useful when coupled with a cluster-aware file system, like GFS and others.  Active/Active setups are where CLVM is most useful.  Active/Passive setups can get away with a little less (http://sourceware.org/cluster/wiki/LVMFailover).

 

In active/active environments, the concept of CLVM becomes even more important when dealing with more complex RAID.  For example, LVM mirroring must be handled in a different way depending on whether a logical volume is clustered or not.  (This is handled transparently by (C)LVM.)  This is because mirroring creates its own metadata to track the degree to which the component legs are in-sync... this status tracking must be cluster-aware or the state will be corrupted.  This is the reason why there is no snapshotting in CLVM yet.  The cluster-coherent version has not yet been written.

 

 brassow

 

On Feb 7, 2008, at 10:36 AM, Gerrard Geldenhuis wrote:



Thanks, the “missing snapshotting” would be a problem. I am still slightly unclear as to the goals of the clustering in LVM…

 

Regards

 


From: linux-lvm-bounces redhat com [mailto:linux-lvm-bounces redhat com] On Behalf Of Jonathan Brassow
Sent: 07 February 2008 16:21
To: LVM general discussion and development
Subject: Re: [linux-lvm] Docs on clustererd option of vgcreate

 

The reason the results are likely so sparse is because the two modes of operation (single machine and cluster) are so similar.

 

Once you've setup a cluster and installed the lvm2-cluster rpm*, new volume groups that are created automatically receive the "clustered" attribute.  [Toggle the cluster attribute by doing vgchange -c[ny] <vg>]  The lvm commands stay the same as if you were running them on a single machine.**

 

 brassow

 

* Of course you could compile the sources or use another install method.

** Some targets (like snapshots) are not available when using LVM in a clustered mode.

 

On Feb 7, 2008, at 8:38 AM, Gerrard Geldenhuis wrote:




Hi

I am after a bit more documentation about the usage of the

--clustered option in vgcreate.

 

At the moment I don’t have a spare san drive with which I can experiment. My understanding is that you would set this flag when different hosts(nodes) connect to the same storage area typically a san with lvm configured.

 

I am after a bit more information describing this “clusterering” functionality in lvm and how/where it is used. The man pages on lvm and commands is a bit sparse and I am trawling through google results but not with great success at the moment.

 

Regards

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_______________________________________________
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read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/

 


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