[CentOS] Success moving Xen LVMs from 32 to 64bit host

[Pasi Kärkkäinen]

On Mon, Mar 01, 2010 at 02:47:23PM -0500, Kwan Lowe wrote:
> Just wanted to share some success I had moving some Xen guests from
> one server to another.
> 
> Problem Recap
> We had Xen host on a single core 32-bit CentOS 5.4 installation on an
> AMD Athlon 2.1 GhZ system that was giving hard drive errors and needed
> to move the LVM-backed Xen images to another server. The replacement
> server was a quad-core AMD Phenom system running 64-bit CentOS 5.4.
> 
> Our original plan was to use LVM snapshots so that we wouldn't need a
> maintenance window. This worked fine in test, but we decided to bring
> down the Xen guests after all.  After shutting down the systems we
> backed up the LVMs.
> 
> To show the backing LV:
> lvdisplay /dev/rootvg/xm_c32_001
> 
> >From this, we grabbed the "Current LE" field and LV Size. We used LV
> Size to create a temporary mount point. We used the "Current LE"
> parameter to create an identically sized LV on the replacement server
> .
> 
> On the failing server:
> dd if=/dev/rootvg/xm_c32_001 of=/mnt/backup/xm_c32_001.out
> 
> We gzip'ed the resulting .out file and saved it as a backup.
> 
> +++Footnote
> BTW, there are many recommendations to do the following on the virtual machine:
>    dd if=/dev/zero bs=1024 count=xxxx of=/partition.out
>    rm partition.out
> 
> By creating the large empty file on each partition  *in the guest
> istance* (/var, /, /home, etc.), it will improve the image
> compression.  Space was not much of a concern and we were worried
> about blowing out a production system, so we opted not to do this.
> +++Footnote
> 
> Once the backing LVM was created, we created the LV on the replacement server:
> lvcreate -l xxx -n xm_c32_001 rootvg
> 
> 
> Then used dd to recreate the file:
> dd if=xm_c32_001.out of=/dev/rootvg/xm_c32_001
> 
> Next, we copied the /etc/xen/xm_c32_001 configuration file to the
> replacement server.  We generated a new UUID using the "uuidgen"
> utility. We also created a new MAC address.  

           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

> Finally, we started the
> instance:
> 
> xm create xm_c32_001
> 
> Everything came up, but no network.  From the root console we logged
> in then edited the /etc/sysconfig/network-scripts/ifcfg-eth0.  Xen had
> apparently renamed the script and put in a DHCP configuration. 
>

It's not Xen renaming the script :) You changed the MAC address, so the
centos network init scripts will rename the ifcfg file, and generate
new default one (with dhcp).

rhel/centos ifcfg-eth* are based on MAC addresses.

-- Pasi

> We just
> renamed the backup file and commented out the MAC address line and
> restarted networking, *and* ifdown eth0 then ifup eth0.
> 
> It took a few seconds for the network to properly discover the new MAC
> address. Once that was done, everything worked beautifully.
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