Re: Ang: Re: clustering with RHEL

How about my original question (it's at the top of this thread)?  CSM
on a shared disk + private segment container + linux HA

In theory it seems the safest way to have a failover storage but, has
it been tested before, is it worth trying?

Regards

On 4/24/07, Steve Dobbelstein <[email protected]> wrote:
>
>
> [email protected] wrote on 04/23/2007 01:39:16 PM:
>
>  > Hi, Steve!
>  >  It seems to me, that it would be possible to use GFS to handle the
>  > access to shared disk. I don´t see the need for EVMS to communicate
>  > with GFS? The only thing EVMS have to be responsible for, is to see
>  > that it exists shared disk from a shared container. GFS will handle
>  > all the mechanisms needed for locking and unlocking. What do you think?
>  >
>  >  Failover of non-shared disk, though, seem to more difficult to achieve.
>  >
>  >          Rgrds Johan
>
>  Hello again.
>
>  You certainly will rely on GFS to handle the coordination/locking of the
> shared volume when the system is up and running.  EVMS does not handle
> runtime issues; its focus is configuration.  What EVMS's clustering support
> gets you is coordinated configuration changes so that all nodes in the
> cluster see the configuration changes at the same time instead of having to
> go to each node manually and have it discover the changes.
>
>  In your case I would recommend not using the Cluster Segment Manager since
> it relies on being able to communicate with the cluster manager.  (If you
> assign the CSM to a disk you will not be able to manage volumes on the disk
> since the CSM will not let you see them because it can't talk to the cluster
> manager.)  You would simply just be careful how you manage your shared
> storage as you most likely do today.  Any physically shared disk will appear
> on all nodes in the cluster.  EVMS on every node will see all of the
> physically shared disks.
>
>  For disks that you truly want shared you can use EVMS to manage the disk,
> but after you make any configuration changes you will want to make sure
> those changes are reflected on the other nodes in the cluster.  One way to
> do that is to run EVMS on the other nodes to discover the changes and then
> "Save" them to activate the new device nodes.
>
>  Physically shared disks that you want to be private to a particular node
> will appear on all nodes in the cluster.  EVMS on every node will discover
> any volumes on the disk and create device nodes for them.  You simply don't
> use the device nodes on the nodes in the cluster that don't own the disk.
> You do, however, run the risk of having the device nodes available on the
> nodes that don't own the disk.  Some errant software or an administrator
> error could cause the disk to be accessed from a node that doesn't own the
> disk.  This is no worse than your current setup.
>
>  Failover of non-shared disks without the CSM shouldn't be too difficult.
> Since every node will be able to see all the non-shared disks and the
> volumes on them, failover is as simple as mounting the file system on the
> backup node.
>
>  The trick is what happens when the node that died comes back online.  You
> don't want the node to mount the file system while it is mounted on the
> backup node (unless it is a distributed file system that can handle multiple
> mounts).  The clustering software should have some hooks for when a node
> comes back online.  You should be able to use the hook and unmount  the file
> system from the backup node before the failed node comes back online.
>
>  Hope this helps.
>
>  Steve D.
>
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