I started a backup on a suspended VM and after a while, since the backup was still running I figured I could just power on the VM and get on with my work.
However I got an error during power on saying
"Cannot open the disk XYZ or one of the snapshot disks it depends on."
Is there a way around it? some option for the backup maybe?
This case falls outside what we consider "reasonable procedures". Backups are based on snapshots, you need a way to freeze the VM so that you aren't copying data from a disk that is in turn being written to. Snapshots is one of the techniques to produce consistent backups, from our point of view it's the best one and that's why we use it.
If you suspend a VM you make a copy of its memory contents to a file and you suspend -may you allow the repetition- the I/O operations on the VM. Then you tell (c)XSIBackup to perform a backup on it. (c)XSIBackup sees the VM is suspended and I/O operations are suspended too, thus it sees no reason to take a snapshot, as the VM is already in a frozen state.
Then you change the rules of the game in the middle of a backup and decide to turn the VM on. You haven't taken a snapshot though, thus the host service tries to access some files which are in turn already open by (c)XSIBackup and you get an error saying that you can't access those files.
Something similar would happen if you try to turn on a VM which is being replicated to or if you try to copy a folder from a VM that is switched on.
Forgive us for being a bit sardonic on this question: this is the equivalent to trying to get down of a car without having stopped it first.
While I understand the "while car is running getting down of it is stupid" statment, a suspended VM is not actually running.
Plus, you can make a snapshot of a suspended VM.
So adding all this together, is not that unreasonable for one to assume that a backup software that is capable of taking a hot backup of a running VM without disturbing it, should be able to handle backing up a suspended VM that later will be powered on.
I will consider this a limitation of the software, and I will schedule my backups of "work VMs" to be done during my sleeping hours, that way there is little chance to power them on. Although every now and then it happens that I work through the night, but I guess I can deal with that if/when that happens.
We could very easily make (c)XSIBackup take a snapshot on a stopped or suspended VM too, or even make it a configurable thing. We are just setting some basic thing: "the rules of the game are not going to change amidst a backup cycle", which is, from our perspective, something not only reasonable but desirable in a hypervisor meant to run servers.
Thus, our software is perfectly able to do what you want. Just keep your VMs on at all time. That way you will follow the basic and most simple rule: not changing the VM state in the middle of a backup job. You will be able to use it while it's being backed up without any problem.
What you are claiming is something totally different which has nothing to do with the ability to perform hot backups, namely: being able to change the state of a VM in the middle of a backup.
And the answer to that statement is very simple and straight: no, we don't offer that feature nor do we pretend to.
Would it be interesting to offer it?: well, that would be a difficult rethoric question to give an answer to. I guess there should be a previous question to place ourselves first: is there some good reason for it?, is there some significant number of users asking for that feature? or at least some case scenario or common situation in which that would be useful.