[PVE-User] Backup Image Size

Flavio Stanchina flavio.stanchina at ies.it
Tue Sep 11 15:09:58 CEST 2012

Christian Cartwright wrote:
> Thank you for the reply.
> The disk image is 250GB
> Windows reading it as 250GB with 207GB Free, then when its backed up Proxmox Backup image is 70GB.

One should never overprovision disk space for a virtual machine (or at 
least not by as much as you did here) in the off chance that someday one 
might need it. The operating system in the VM will happily sprinkle 
blocks here and there sometimes, but it doesn't fill those blocks with 
zeros when they are no longer in use; those blocks will not compress 
well and you'll end up with an unnecessarily large backup, as you just 

It's easy to enlarge the virtual disk or add a new disk --even 
temporarily -- when needed. With Proxmox VE 2.x you can even exclude a 
virtual disk from backups -- perfect for temporary storage space in the 
VM, just make sure that the users know which disk is safe and which one 

> I will certainly try SDelete!

Careful with tools that zero the unused portion(s) of your disk(s): 
while it will be beneficial for backups, it may also happen to write to 
*every* block of your virtual disk and if you're using thin-provisioned 
storage (like qcow2, vmdk, or ZFS thin-provisioned vdisks via iSCSI) you 
might end up filling it to capacity, depending on how smart it is in 
handling zero-filled blocks.

Flavio Stanchina
Informatica e Servizi
Trento - Italy

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