Disable the QEMU agent on the VM options, that'll allow the backup to run. I'm not sure if the issue is Debian or the MariaDB builds.
If you could kindly report upstream links here that would be great, as I would love to get this fixed.
So it effects 10.3 as well.
Please report this upstream to mariadb as well https://jira.mariadb.org/browse/MDEV-27196
Also upstream to qemu project https://gitlab.com/qemu-project/qemu/-/issues/881
At the moment we don't know which side the issue lays with, more people who report it, more...
I have the exact same problem, Debian and MariaDB from their own Repo.
Only setup the VM last week, so far no other Debian VM have been effected. I did reinstall the agent on the MariaDB VM, which got it working for a while but eventually issue comes back.
You can try downgrading qemu as the new qemu 6.1 that has shipped with PVE7.1 maybe what's causing you issues
# Download old qemu
# Install old qemu
Issues with Virtio IO errors in guests have been reported on Bugzilla and upstream on QEMU. The issue doesn't look to be isolated to just Proxmox.
You'll also want to amend the default postfix configuration so Proxmox can send emails, you'll want to create a GMAIL account or something to send as. Otherwise email delivery probably won't work.
Create a gmail account called like...
That strategy is perfectly fine, just means more downtime as all. I assume its a personal setup so you'll be fine.
Don't forget to monitor yours disks with smartctl or something, so you get alerted hopefully before they fail.
EDIT: if you setup your install using ZFS with a single drive, I...