Hyper-: Disk Full ?

[TheOnlyWizard17]

Hi,

We've got a 2019 Hyper-V Host acting as storage for Veeam. Veeam is installed as a 2019 VM ontop of this host, and has 2 volumes, combined as Extents for a SOBR. This VM thus has 2 FIXED VHDx, each 43,6TB in size, which fit roughly onto the underlying storage of 87,3TB. 2x 43,6=87,2TB, so this means there's 100GB left on the host-volume. Both the Extents inside the VM and the storage on the host are ReFS 64K. It has bothered me since that that storage says it only has 2MB free, while if you add up the byte-numbers from VHDx's and total, used, free space from that host-volume, you calculate ~100GB free. Our Extents since have been receiving backups and are now roughly half-full. Last week, our VM suddenly started "failing", e.g. it went into Paused-Critical-mode. Reason Hyper-V gave was: Disk Full, referring to the host-volume. And indeed, that volume (the volume which SHOULD have 100GB free) was saying it had 0 bytes left. I waited for like 15 minutes, checked again, and it had 1,6MB free. So I resumed the VM and "nothing" happened. This happened a few times during the course of that backup-round. When the backups were done, free space gradually increased on the host-volume to about 1,9MB and stays there. Next evening, same ritual. From that, we decided to reduce retention from 31 days to 14 days, even though the fixed VHDx's still had roughly half of their capacity free space. Veeam went about it reducing retention by merging older incrementals (Fast clone), and the issues where the VM got paused still occured during that time. After that was done, backups now run smoothly again and VM doesn't get paused any longer. So big question here is:
Has anyone encountered a similar situation or does anyone have any clue why a host-ReFS seems to be needing free space to accomodate writes inside 2 fixed VHDx's which ofcourse should not be writing ata all outside of their file-boundaries ?

[HannesK]

Hello,
To me it looks like it's how Hyper-V handles writes. I assume that it has nothing to do with Veeam. Did you try something similar without Veeam? What does Microsoft say on that topic?

Having only 100 GB free on a 87TB volume sounds too little for my "gut feeling" (no matter which hypervisor and whether thick or thin provisioned).

Best regards,
Hannes