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Re: [Xen-devel] Migration of block devices

To: Rob Bradford <rob.bradford@xxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Migration of block devices
From: Jacob Gorm Hansen <jacobg@xxxxxxx>
Date: Fri, 04 Aug 2006 00:09:25 -0700
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On Thu, 2006-08-03 at 15:08 +0200, Rob Bradford wrote:

> One approach to achieve this I have heard mentioned is to proxy the
> local interactions with the backing block device through a degraded
> RAID1 array (using software RAID). The destination server then exports
> its local destination block device (e.g. loopback file or LVM) via NBD
> (network block device). This block device is hot added to the degraded
> array and the data is synced from the original block device to the
> destination. When the domain is migrated (using xm migrate) it can then
> use the local block device from that point on. In order to allow future
> migration from this point on the virtual machine must still access the
> block device here via a proxying degraded RAID1 array.
> Has anyone successfully done this? My initial attempts seem to show that
> it should work in practise but I experienced some problems with
> corrupted metadata in the filesystem.

Yes I have, but that was a couple of years ago, using self-migration. It
worked well back then, except, as you describe, that I had no way of
'hotswapping' from iSCSI to the local Xen-backed version of the disk
device. Also, my minimized boot loader did not have disk access, so I
had to rely on an external domain providing the iSCSI service.


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