RE: [Xen-users] migrating with physical device as VBD

["Ross S. W. Walker" <rwalker@xxxxxxxxxxxxx>]

Jan Schulze wrote:
> 
> Hi all,
> 
> I am wondering, whether it's possible to migrate a DomU from one XEN Host to
> another *without* using shared storage. All HowTos that I found so far, are
> using some kind of shared filesystem or block device.

Shared storage is mandatory, but it doesn't have to be traditional multi-host
scsi or fiber channel, with technology like iscsi and software like drbd
shared storage can be done on the cheap.

> The DomU in question is using a physical device (partition /dev/sda1) that is
> exported as VBD from Dom0. If migration is possible with that setup, where 
> will
> the DomU end up on the target system?

It needs to be accessible there too at the same time, thus why shared storage
is mandatory. Check out drbd, the latest version does primary-primary which
is what you need for live migrations.

> In case, migrating this DomU won't work, I would have to copy the VM manually.
> Is it possible to access the contents of /dev/sda1 from Dom0? Simply shutting
> down the DomU and mounting /dev/sda1 in Dom0 does not work.

There will be a partition table within that partition, so you can run
kpartx -a /dev/sda1, and access the first partition via /dev/mapper/sda11,
or mount it via a loopback mount to the offset of that partition.

# mount -t <fstype> -oloop,offset=<part offset> /dev/sda1 /mnt

-Ross

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