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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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