Synology Volume Reporting 100% Disk Space Used
So I had a weird one for a while which I never noticed until it all came to a head. Primarily, my issue was that a volume was exceeding 10TB in space and I always assumed that was fairly normal for the data I was trying to transfer over to it, however I started to question the legitimacy of my disk usage when I added an extra 10TB and it filled up seemingly overnight.
Why Synology can report 100% Disk Used
Tertiary Google Searches tell me that issues similar to mine can be caused by any application which uses snapshots, versioning, trash cans or anything where changes are archived until no longer needed.
If you're experiencing this issue I highly recommend having a coffee and truly looking at the apps that you have installed, because they'll have their own isolated settings you may not think to check.
My Case: Snapshot Replication
We use snapshot replication to take snapshots of our backups and transfer them to an off-site location. This allows us to recover from minor disasters - should we need to - and to recover data when users make boo-boos. Snapshot Replication doesn't just replicate snapshots; it also does the snapshotting.
The problem for me is that it was set to 'always retain' which explains why every time I tried to change the data (ie delete) in the directory, I would be met by an increasing volume as opposed to the expected inverse result.
The default "Number of snapshots to retain" was 1024, which I found semi-humorous is a very dark manner, so I set that setting and adjusted the retention to just 2. This means I have a semi-backup to catch my own fuck-ups however I won't be using 1024x my disk space (dont quote me on that lol).
Over the hour after setting this, I can see my NFS-mapped Shared Folders from that volume increasing in free disk space - from 100%, to 99% instantly, to 90% after a few minutes to 79% after just under an hour - and it keeps going.
Fingers crossed, I can finally take my backups fully after all these months.