It looks like it's not just XCP-ng Center / XenCenter that create snapshots with the is-a-template parameter set true. The xe CLI does as well when creating a snapshot with the xe vm-snapshot command and some xe commands like xe vm-list show the snapshots with is-a-template set false but not the ones with is-a-template set true. xe snapshot-list shows both types but allows you to select just one or the other with is-a-template=true (or false). xe template-list doesn't seem to allow showing the snapshots by trying to select using is-a-snapshot=true though the parameter is accepted and not flagged as an error.
Odd behavior all around. I only noticed it because I have scripts that parse through the vm-list output and end up catching XO snapshots but not others in the process.
I was originally thinking this might be a bug in XO but maybe it's actually a problem in the xe vm-list command or XAPI in general, i.e. a list of VMs shouldn't list snapshots the way it already doesn't list templates.
@olivierlambert Thanks, I tried running fstrim but it returned "the discard operation is not supported" on this NFS SR mount. I also tried running on SSD mounts which do support TRIM and they all returned the same message about "discard...not supported."
The VM OS (CentOS 7.8) does support TRIM in kernel so not sure if xcp-ng or xenserver is the reason why the VM can't issue fstrim on the NFS SR mount or the SSD SR mounts.
So far, I noticed this problem for three VMs. Each time I am trying to perform a backup with zstd compression through XO, the backup is marked as successful, but output XVA file is damaged (incomplete). The backup works fine for all other VMs, and I cannot pinpoint what is special about these faulty VMs.