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RE: [Xen-users] Xen VMs on GFS volumes

To: Anders Bruun Olsen <anders@xxxxxxxxxxxxxxx>
Subject: RE: [Xen-users] Xen VMs on GFS volumes
From: PAINCHAUD Christophe <cpainchaud@xxxxxxxxxxx>
Date: Fri, 9 Dec 2005 10:28:24 +0100
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>How about live migration, is it okay to just have the blockdevice
>available on both machines at the same time, or does that require a
>clustering filesystem?

That's what I am using : I dont have iSCSI devices, so I am using GNBD (part of redhat GFS/Cluster suite) to share block devices between all dom0. It works almost the same as your iSCSI configuration.

>Again, my setup is with an iscsi storage box (an IBM DS300) which both
>machines has access to. If I don't run a clustering filesystem and mount
>the same device on both machines, I run into trouble (obviously). But I
>am not sure if a partition containing a VM actually needs to be mounted
>two places at the same time, or if it just really quickly unmounts on
>the first dom0 and mounts it on the second?

You don't need to mount any filesystem at all in dom0. dom0 needs to mount them when you are using loop devices (ie: a big file on dom0 mounted filesystem that you are using as a block for your domU)

All what you need is to import your iSCSI block devices on all your real hosts and make sure they all have the same name on all of them (ie: /dev/myISCSI/vm001 ...).

When you will start your VM on one of your dom0, it will share the block device with the domU. The domU itself will mount it. Then if you migrate your domU to another real host host, it will be paused, IO flushed and its memory and cpu registers transferred to the new host and unpaused, then it will resume its disks operations there. On the old real host, all resources have been freed so nothing should use the block device anymore.

Also I have noticed when I ran the same VM twice on different real hosts sharing the same block device (formatted with ext3), that the second VM won't boot until the first one has released the filesystem. It looks like EXT3 got a little mechanism that avoids you to mount it unless you use the force options (which a normal boot doesn't do). So if you are using EXT3, and you make a mistake and run same VM on different xen nodes, there is a good probability that you won't break anything.


Christophe PAINCHAUD

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