Old chunk files not removed after prune + GC on Proxmox Backup Server

[mohnewald]

Hello everyone,

I’m seeing some unexpected old chunk files on my Proxmox Backup Server, even though prune and garbage collection jobs are running regularly.
My prune configuration is:

Daily:   7
Weekly:  5
Monthly: 2

Today is November 13th, 2025, and the last garbage collection ran successfully this morning and removed 50GB.

However, I still see chunk files from February:

root@proxmox-backup:~# ls -lah /mnt/backup-cluster1-ssd/.chunks/0480/048057471f15e4d5ffb8b762ab599b1599d4b0b74ab5d5b7a2c268582277a1d6
-rw-r--r-- 1 backup backup 3.7M Feb 14  2025 /mnt/backup-cluster1-ssd/.chunks/0480/048057471f15e4d5ffb8b762ab599b1599d4b0b74ab5d5b7a2c268582277a1d6

Inspecting the chunk shows this:

root@proxmox-backup:~# proxmox-backup-debug inspect chunk /mnt/backup-cluster1-ssd/.chunks/0480/048057471f15e4d5ffb8b762ab599b1599d4b0b74ab5d5b7a2c268582277a1d6
CRC: "4171794187(OK)"
encryption: "none"
is-compressed: true
size: 3872581

My assumption is that this chunk might still be referenced by a previously deleted or unknown backup (for example, an old VM that no longer exists).
Or did this File never change sice Feb and there is no need to create it again?

Is there any way to check which snapshot or client still references this specific chunk?

This did not return a match:

find /mnt/backup-cluster1-ssd -type f \( -name "*.fidx" -o -name "*.didx" \) \
  -print0 | xargs -0 grep -l "048057471f15e4d5ffb8b762ab599b1599d4b0b74ab5d5b7a2c268582277a1d6"

Cheers,
Mario

 

[Chris]

Check the chunk files access timestamp in the output of stat <path-to-chunk>. If this is newer than the start-time of the latest garbage collection, then the chunk is still referenced by an index file.

Chunks do need to be written only once, as since their digest uniquely identifies their content, there is no need for them to be rewritten. (PBS only makes an exception in case the chunk has smaller size, meaning better compression).