Re: [Bacula-devel] Sparse file/device handling: Just dangerous, or really buggy?

[Bastian F. <bas...@co...>]

Am Dienstag, 24. Juli 2012, 10:27:21 schrieb Josh Fisher:
> On 7/24/2012 9:28 AM, Bastian Friedrich wrote:
> > 
> > we are using Bacula's raw block device backup features for backing up
> > virtual machines that use raw devices (logical volumes, or DRBD) as their
> > storage. As advised by the bacula documentation (obviously, for the
> > purpose of sparing storage space), we set "sparse = yes" for the
> > respective file sets.
> > 
> > Recently, we discovered a problem with systems backed up in this way.
> 
> That is only one of the problems with backing up VM images. Backing up
> only the entire VM image is only useful for disaster recovery purposes,
> as it makes it impossible to restore selected files inside the VM.

In most cases, it is possible to mount file systems of restored disks without 
restarting the complete VM. Granted, it is still required to restore the whole 
disk, even if only very few data are required.

> Nothing can be restored at all without taking the VM offline.

Au contraire, it is well possible! However, doing so is not always simple.

We use/offer two mechanisms, the first being mounting restored file systems, 
while the second is running restored VMs in something like a quarantined 
environment (so that they can run simultaniously to the original VM).

> In an HA
> cluster environment, some mechanism must be employed to determine which
> cluster node should be backing up the VM image.

Depending on the setup, yes. We use one (of usually no more than two) node for 
backup purposes. Yes, this results in a single point of failure for the backup 
service (which is then not run in HA). Has never been a problem so far.

> Really, the only
> advantage is that it reduces the size of the Bacula catalog database and
> *may* reduce backup time.

The most important point is that you don't need to install and maintain client 
components ("agents" in other backup software's lingua) on the virtual 
machines.

I'm not at all saying that block level backups for virtual machines are in any 
way superior to a file level backup within the machines. Both have unique 
advantages and disadvantages, and we offer both; however, as a matter of fact, 
virtualization customers -- our customers -- show a large demand for block 
level VM backups.

Thx anyway!
   Bastian

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