Zero-Downtime Limited-Space Backup



This is yet another backup script.  We use it internally, but we thought other organizations might find it useful.  It is based on using the python API and command-line LVM snapshots.

We wrote this because other methods didn’t quite meet our needs:

1) No downtime – Doing exports requires a shutdown.

2) Limited space – Doing built-in snapshots of VM’s was not feasible for us.  Currently, there is no way to exclude disks in a snapshot (that we have found).  A snapshot will take about double the currently used space for a disk on an SR, and this space cannot be reclaimed until the snapshot is deleted and the machine is shutdown to be coalesced.  In one of our VMs we have about 8 TB of user drive space, with no extra space on the SRs where the disks are allocated.  We don’t have enough room, nor do we care about creating a snapshot with the user data since it is already backed up with netbackup.  The script allows us to get no-downtime snapshots of the system disks with only requiring a small and temporary amount of extra space on the SRs.

How it Works

The python API is used to gather metadata about the VM, its disks, and its network interfaces.  The metadata is written to plain text files.  The data from the disks is imaged by doing a dd on the lvm volumes that correspond to the VDIs for the disks.

To restore, a new VM is created and given the memory and CPUs settings stored in the metadata.  Then the VIF and disks are restored with the stored images being written to the new lvm volumes.

Work in Progress

The script is still a work in progress and, while it fits our needs, does not have all of the features it could (Comments welcome).

For instance, It does not currently work for HVM guests.  We used an ugly (but quick) kludge where, for a new VM that we are restoring, we just copy an existing VM that we know has booted before, but does not have any disks.  This allows us to avoid the VM create command (just look at it).


1) Create a config file like the example given changed for your environment.

2) (Kludge) Take a working linux vm you have now, copy it, and remove its disks.  Give it a unique name and put this name into under copy_vm.



Interesting Snapshot method.

Leave a Reply

Your email address will not be published. Required fields are marked *