That is from the Storage Dashboard. If I have a mirror comprised of two nominally 16-TB drives (shown as 14.55 TiB) my usable capacity should be 14.55 TiB, not 27.07 TiB, as shown. Am I missing something?
I am guessing you have Block Cloning involved since Used is 17.92 TiB. Was there a lot of copying or moving data between datasets?
I am using that volume as the target volume for backups done by UrBackup. It would make sense that what I understand block cloning to be, would be involved, in order to save space between the backups of large files with minimal differences. My UrBackup configuration does both image backups and file backups.
Would ZFS deduplication do its work on the block level? In that case would that lead to block cloning?
So, if your hypothesis correct, then the total storage consumed by the Storage Dashboard is computed by adding all the file sizes, rather than the number of occupied blocks multiplied by the block size. Am I stating that correctly?
If so, the usefulness of that graph is very limited. What I really want to know is, at what point I have to replace my 16-TB drives with 20-TB drives, because my storage is running out.
I filed a bug report on this: NAS-139974.
A pet peeve of mine: The usage of the word “usage” on that dashboard display is incorrect. The word “usage” is used when one talks about how something is employed, in order to achieve its purpose. A better word would be “utilization.” I also filed a bug report on this second issue: NAS-139973.
I was directed to post this to the Feature Requests category in this forum.