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As I understand it, the only way to do a live backup of a vm in XenServer is to make a snapshot, convert the snapshot to a template, then export the template (to another server), then delete the snapshot.

What I can't find anywhere are the specs for how much space a snapshot takes, or how they work, plus I've had conflicting results from my trials so far (more on that below if anyone is interested). Are they supposed to be full copies right from the start? Or do they work on some sort of copy-on-write algorithm? Meaning they start at 0 size, and then grow over time (so if I delete them immediately after the export, I really won't need much disk space at all).

That's my question - how much free space to I need to leave on a host in order to do backups of the vm's this way?

Some more details on why I'm so confused (other than by the complete lack of documentation by Citrix on this topic):

Our issue is that space is at a premium - our disks are not huge on the hosts - only a little over 200GB each (using ssd's).

On one host, I get an error "The specified storage repository has insufficient space" when trying to take a snapshot of a 20G vm ... this host has 4 vms currently on it (100+20+20+20=160)

Yet on another host, with 2 x 100G vm's I was able to take a snapshot of one of the vm's.

I just noticed something odd in the storage tab on xencenter: Name / Type / Shared / Usage / Size / Virtual Allocation Server1 / LVM / No / 98% (205.4 GB Used) / 207.6 GB / 200.4 GB Server2 / Ext3 / No / 59% (120.9 GB Used) / 204.3 GB / 300 GB

I should note, there are no snapshots of any other vms than what I mentioned. So the first server has got 40G too much in 'virtual allocation' ... the 2nd server allowed 300 to be allocated out of the 204 available? And only saying 120 is used?

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