Vm in readonly mode when backup is running?

[antonov314]

Hi,
If I create a backup of a running VM in Proxmox, and the backup task is running, does the VM get put into read-only mode during the backup?

For example: if the VM is running a database that is constantly processing data, will the database continue to receive new data during the backup, or does that stop while the VM is being backed up?

Hope my question is clear, sorry if its not.

 

[aaron]

When VMs are getting backed up with Proxmox VE native backup methods, VMs will continue to fully run during the backup.
3rd party vendors can utilize the public backup API. Then things work the same. If they use a custom implementation under the hood, things might change.

But depending on how fast the backup data can be written to the target (network speed, write performance on target, ...) it might affect the write performance during the backup.
This is because we want the virtual disk in the state it was at the beginning of the backup. So if the VM wants to write to an area of its disk that hasn't been backed up yet, PVE will temporarily block that write op. In the next step it will jump to that area of the virtual disk and back it up out of order. Once the data in that area is safely written to the backup target, the block will be lifted and the write operation from the VM will finish.

If that affects your VM performance too much, take a look at the Fleecing property for backup jobs. This way, a (hopefully way faster) local storage will be used to write the data to in the first place, before being sent to the backup target. This way the block for the VM will be a lot shorter.