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[Xen-users] Backup DomU volumes

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Subject: [Xen-users] Backup DomU volumes
From: Pavel Georgiev <pavel@xxxxxxxxxxxx>
Date: Tue, 2 Oct 2007 16:40:48 +0300
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Hi List,

I`m trying backup the volumes of VMs running on a xen server (3.1). I`ll 
backup ext3 and reiserfs volumes. My goal is to backup them from Dom0 so that 
I can avoid messing with DomU (adding scripts, cronjobs etc.). I have another 
server running xen and I plan to keep exactly the same volumes on it so I can 
start the VMs on this server in case my main server dies. 

The way I see it, I`ll be mounting volumes on both servers and 
doing 'rsync -avP' on the content (in order to avoid transferring already 
existing file). The problem (maybe not a problem of xen, but I guess other 
people have bumped into this) is how do I mount volumes without stopping the 
VMs and without corrupting the data on the volumes. 

When a volume is in use by a VM, it either has the "need_recovery" flag (ext3) 
or cannot be mounted ro (reiserfs). I could remove the "need_recovery" flag 
for ext3 volumes and mount them read-only, do the sync and then umount and 
set the flag again. For reiserfs I could mount the volume read-write, remount 
read-only and then do the sync. Are those two approaches safe? Is this going 
to corrupt my volumes, after all I`ll do some writing (removing the flag for 
ext3 and replaying the journal for reiserfs)?

In general, what are my options if I want to copy the volumes to another 
server (hopefully only the changed files) without stopping the VM. The rsync 
does not have to be atomic, it is OK if DomU writes some data after I start 

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