[linux-lvm] Docs on clustered option of vgcreate

Thu Feb 7 17:26:10 UTC 2008

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 at redhat.com [mailto:linux-lvm-bounces at 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 at redhat.com [mailto:linux-lvm-bounces at 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

_______________________________________________
linux-lvm mailing list
linux-lvm at redhat.com
https://www.redhat.com/mailman/listinfo/linux-lvm
read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/

_______________________________________________
linux-lvm mailing list
linux-lvm at redhat.com
https://www.redhat.com/mailman/listinfo/linux-lvm
read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://listman.redhat.com/archives/linux-lvm/attachments/20080207/b5409a66/attachment.htm>