Am having concerns about used space on my RN3138. I have it fully populated with 4 10TB WD Gold drives which yields total available storage at 27.27TB. Of that only 7.67TB remains available. I have twin units actually and one performs backups to the second periodically a couple different ways but daily there are snapshots being sent to the remote NAS using ReadyDR for the main part of my data and another volume of shared data is being backed up once a week using the Backup utility under Jobs. So far I've been able to account for 8.163 TB data which is being consumed by a backup series performed from my office server as well as some data that's put on the NAS manually that is shared with the network. I see many snapshots on the NAS but can't determine how much space they are actually consuming on the unit. How can I figure out where all my space is going?
Are quotas enabled at the volume level? If they are you should see how much data is current data vs snapshots.
Note that if you delete a file the space that file consumed will only be freed up if all snapshots containing that file are also be deleted.
If you modify a file then the space consumed will go up as it has to hold old copies of the data.
What does your btrfs.log look like especially the top section above the === subvolume /data === line?
How many snapshots do you have roughly.
What kind of data is being backed up? Is it mainly small files (e.g. word documents, spreadsheets etc.), large files (such as photos, short and/or low-res videos), huge files (such as long hi-res videos, iSCSI LUNs), a mixture?
Is the space available the same on both units or is one very different to the other?
