Usually you would use LVM/thick on a ISCSI or fibre-attached SAN:
https://pve.proxmox.com/wiki/Storage:_LVM
Before PVE9 you couldn't use snapshots on it though. Beginning with PVE9 a new feature was introduced which allows qcow-based-snapshot-chains as "technology preview".
They have some caveats though and are (as said) still in preview status so I wouldn't relie on it for production.
@bbgeek17 wrote some great writeups on using LVM and the snapshot feature, which you might want to read:
https://kb.blockbridge.com/technote/proxmox-lvm-shared-storage
https://kb.blockbridge.com/technote/proxmox-qcow-snapshots-on-lvm
https://kb.blockbridge.com/technote/proxmox-tuning-low-latency-storage
https://kb.blockbridge.com/technote/proxmox-qemu-cache-none-qcow2
Basically it boils down that if you configure everything correctly LVM/thick (with raw instead of qcow) you will give you a rock-solid storage but without thin-provisoning and without snapshots. With the new feature you will have snapshots but they have some caveats ( including a severe performance pennalty) and still need to mature.
For backups however this doesn't matter: The native backup features of ProxmoxVE don't need snapshot support on the storage since the virtualization layer (qemu/kvm) has it's own internal mechanism to snapshot the vm'state which can be used for backup. ProxmoxVE has two native backup features:
- Creating a full backup to an archive file which can be on any supported file system. One popular approach is to use a NAS as target for them. Their advantage is, that you don't need any other software except ProxmoxVE to restore them. Their disadvantage is that they will always full backups so will eat up some space. It might still be a good idea to have some of them (maybe once a month or something like that) for your most important vms, where even an older state is better than none
- Creating a backup to a ProxmoxBackupServer. PBS has a quite smart deduplication mechanism and offers several options to implement ransomware protection and offsite backup (Tape, external USB storage ("removable datastore" or syncing Backups between Proxmox Backup Servers). The last version (4.1.) also introduced support of S3-cloud storage for offsite backup, but it's still in technology preview (so good enough for testing, but nothing for a sole offsite backup).
Since both of them use the internal mechanism of qemu they don't need storage support for backups. The same mechanism can also be used by other backup software to implement ProxmoxVE support. In fact Veeam did this in their ProxmoxVE plugin, so the lack of snapshots is not a problem if you want to continue using Veeam. What would makes me weary, that the reports in this forum (you can search for it) still seems to indicate that it still needs to mature and that the support for application-aware backups (like for domain-controllers or SQL databases) still lacks compared to their vmware integration. Treating the VMs as baremetal servers and installing the veeam agent inside of them seems to work however.
So I would consider switching to ProxmoxBackupServer for your regular VM backup and enjoy it's good integration in the HypervisorUI and just licence the necessary minimum to run Veeam agents for application-specific stuff. Of course depending on your budget and other constraints that might turn out to not be a viable option. But if you need to do a migration anyhow for the hypervisor now why not also rethinking your backup approach?
The BackupServer is (like PVE) completely OpenSource and has no features locked away behind a paywall. You still need to pay to disable the nag screen, get access to the enterprise repo and payed support, but for doing an evaluation you don't need to pay a penny.
