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Re: [Xen-users] Backup domU

On Sat, 2011-07-09 at 08:54 +0100, Simon Hobson wrote:

> One suggestion given here in the past was to send a signal (with an 
> xm command IIRC) to the guest (assuming guest with Xen support 
> included) to tell the guest to sync it's unwritten buffers to disk. 
> He then did a snapshot live via LVM.

I have been exploring this myself. I started out by pausing the guest
and copying its image. That of course gives one of those semi-corrupted
images that you would get from pulling the plug on a real machine, plus
it causes a temporary service outage on the guest. To avoid the service
outages, I switched to using an LVM snapshot (I had to rearrange a lot
of things to make this work, as you have to have extra space on the LVM
volume to allow for snapshots and I didn't have this initially). That
works and eliminates the service outage on the guest, but still produces
a semi-corrupted image. 

So after reading this (and taking a vacation), I did some more reading
and discovered that using "xm sysrq name-of-guest s" will cause the
guest to sync, assuming that the guest has been set up to allow this
(kernel.sysrq = 16 in the sysctl.conf file). So I tried syncing the
guest this way and then immediately taking a snapshot:

xm sysrq name-of-guest s; vm snap

("vm" is a script I wrote that sets up and mounts the snapshot, among
many other possible operations). This is going to create the snapshot as
soon after the sync as is realistically possible.

The /var/log/messages file on the guest shows that the sync did occur.
But if I run 'file' on the guest image from the snapshot, it still says
that the ext3 file system "needs journal recovery". So I am concerned
that I may still be getting a semi-corrupted image. Is there a way to be
certain that the image is clean? Is there a way to force the guest to
run the journal as well as syncing?


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