[ale] Quick backups of Xen domU filesystems

[Jeff Hubbs]

  I'm setting up a Xen server for hosting of virtualized internet-facing 
servers and I'm looking for a way to make quick backups of the largish 
domU filesystem files on the physical host.

Right now I've got an extendable LVM2 logical volume (now 32GiB in size) 
set up specifically for holding these files and I expect each one to be 
at least a few GiB in size.  I know that LVM2 has some sort of 
snapshotting mechanism available but I'm not sure if it really does what 
I want...I'm envisioning some sort of write-twice filesystem or 
abstraction such that if the source volume is made stable (i.e., all 
domUs are stopped and all files are closed), the snapshot copy can be 
very quickly finalized and frozen so that the source volume can be put 
back in service right away while tape backups can be made from the 
snapshot.

Because there will be a relatively small number of large and complex 
files in the source volume, I feel like rsync between two ordinary 
volumes would be little better than a simple copy and I'm looking for 
something that allows me to minimize how long the domU files in the 
source volume have to stay closed.  In fact, I would like to give the 
operators of the domUs specific times when their domUs will be shut 
down; if I'm depending on rsync/cp or something slow, in order to 
minimize downtime I'd have to stagger the shutdowns or run them 
serially, thereby getting wishy-washy on the *actual* downtime.

So what modern options do I have for a sophisticated way of doing this?