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Re: [Linux-cluster] RHEL 6 cluster filesystem resource and LVM snapshots



Hi,

1.
I found in the "Logical Volume Manager Administration" manual for RHEL 6 on p.12 and on p.35 the following statement:

"LVM snapshots are not supported across the nodes in a cluster. You cannot create a snapshot volume in a clustered volume group."

I understand that this means no snapshots for shared volumes when using CLVM.

2.
Strangely enough, I also found the following fragment on p.4 of the same manual:

"If you are using a clustered system for failover where only a single node that accesses the storage is active at any one time, you should use High Availability Logical Volume Manager agents (HA-LVM). For information on HA-LVM, see Configuring and Managing a Red Hat Cluster."

I understand this as an older system of synchronizing LVM configuration across the nodes that predates CLVM. I thought it was deprecated. Anyway, I cannot find any pointers to it in the current RHEL 6 manual.

Questions:

- Is HA-LVM still a viable option for the "one at a time" scenario described in the fragment above?
- If so, where do I find more information about HA-LVM? What should I search for?
- If it is still a viable option, would it support LVM snapshots?

Thanks and regards,

Chris Jankowski


-----Original Message-----
From: Jankowski, Chris 
Sent: Wednesday, 24 November 2010 07:23
To: 'linux clustering'
Subject: RE: [Linux-cluster] RHEL 6 cluster filesystem resource and LVM snapshots

Roger,

Thank you.
I see your point.  Indeed, it looks that I need CLVM regardless of the type of filesystem used.

Question:
Does CLSVM support snapshots?

Thanks and regards,

Chris Jankowski

-----Original Message-----
From: linux-cluster-bounces redhat com [mailto:linux-cluster-bounces redhat com] On Behalf Of Roger Pena Escobio
Sent: Wednesday, 24 November 2010 01:32
To: linux clustering
Subject: Re: [Linux-cluster] RHEL 6 cluster filesystem resource and LVM snapshots



--- On Tue, 11/23/10, Jankowski, Chris <Chris Jankowski hp com> wrote:

> From: Jankowski, Chris <Chris Jankowski hp com>
> Subject: Re: [Linux-cluster] RHEL 6 cluster filesystem resource and LVM snapshots
> To: "linux clustering" <linux-cluster redhat com>
> Received: Tuesday, November 23, 2010, 6:45 AM
> Xavier,
> 
> I do not think that I have to use CLVM with ext4 or XFS in
> a cluster.
> 
> The ext4 or XFS filesystems will be on shared (FC) storage,
> but they will be presented as a filesystem resource i.e.
> accessible to only one cluster at a time, as they have to
> be. So, I believe that simple LVM will do and snapshots will
> be available.

Hi Chris

first I would like to say it has been a long time since I used RHCS and the only major problems I had was with clvm, but,  by your question, I think if you still foresee changes in the lvm space, you will still need clvm, even if the filesystem will be a non-cluster FS

the lvm layout is independent of the filesystem used, you might want to add more volumes to a group, resize, etc, and that info is read and cached when the kernel read the device, not when mounting the filesyste, so, if the device is presented to a node of the cluster, it will read the lvm layout and filesystem properties, even if not mounted. If you change that layout in one node, the others nodes might have a wrong information that could led to a crash in case you tried to mount the fs in there.


see the point of having clvm in a cluster even if using ext3/4 ?

if you don't plan to use lvm for the cluster, which is possible since you are having the device from a SAN/NAS/iSCSI where you will have exactly, or almost exactly, the same features that LVM provide, why having the extra layer if you will not use it ?

that is the conclusion I reached years ago when facing problems with CLVM

thanks
roger



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