Alert Less than 20% of volume data's capacity is free. Performance on volume data will degrade

Hi davidk1952 

The disks and raid are healthy, which is good.

I want to point out that the NAS uses Tebibiyes not Terabytes (it's a Linux thing). So, your actual space is 10.9 Tib = 11.98468 TB. This makes sense given you have 4 x 4TB drives in a raid 5 config - i.e. one drive goes to redundancy, leaving you with ~12TB usable space (10.9 TiB).

The NAS makes a raid with a software called mdadm (Multidisk administration) and then it presents the raid as one whole device to the filesystem layer. Therefore, the filesystem is not aware of the underlying raid, in that sense. It also means that the raw 16TB has no place in the space usage calculations. The filesystem sees a device presented that is 10.9 TiB (~12TB) and it created a volume on that device, so from the filesystem perspective that is all there is to it. You have a volume of 10.9 TiB and it looks at how much is occupied by data out of the those 10.9 TiB and there is your calculation.

Looking at the filesystem stats, I can see that you indeed have a volume of 10.9 TiB and you have used exactly 8.64TiB. That is 79.26% space used. This means the warning you are getting is triggered correctly. The filesystem sees that 80% of the space is used and the NAS triggers a warning.

I will mention one thing here. The (BTRFS) filesystem quotas are looking wonky as we have seen before. This can sometimes lead to incorrect capacity calculations by the filesystem. I see references to subvolumes that don't exist and also see nonsense like below where it reports some fictitious subvolumes (shares/snapshots) are 16 Exbibyte in size:

=== qgroup /data ===
qgroupid rfer excl max_rfer 
-------- ---- ---- -------- 
0/262 16.00EiB 16.00EiB none 
0/270 16.00EiB 16.00EiB none 
0/271 16.00EiB 16.00EiB none 
0/1180 16.00EiB 16.00EiB none 
0/1181 16.00EiB 16.00EiB none 
0/1186 16.00EiB 16.00EiB none 
0/1188 16.00EiB 16.00EiB none 
0/1192 16.00EiB 16.00EiB none 
0/1193 16.00EiB 16.00EiB none 
0/4716 16.00EiB 16.00EiB none 
0/4717 16.00EiB 16.00EiB none 

It is obviously nonsense and simply a bi-product of BTRFS quotas being "finicky" and sometimes calculates incorrectly. We have seen this several times before. You read more about it here 

We also had a recent discussion about this topic in this thread 

I would advise that you disable quotas and reboot your NAS. See if you get some "more space". You can re-enable them afterwards but I would suggest to keep them off. The only thing they provide are snapshot and share space consumption calculations for you to see - neither which are essentials.

To disable Quotas, go to admin web page and navigate to: "System" > "Volumes" > Click the cog-wheel next to your raid Volume > select "Settings". Once in Setting, select "Summary" on the left hand side. Here you Un-tick the box that says: "Quota" > Click "Apply" at the bottom. Afterwards, give the NAS a reboot.

Hope this helped. Cheers