Balance failed after adding new disks

Troubleshooting
1

Hello all.

I have a 10 disk raid10 setup with the following disks:

2x1TB
2x2TB
2x3TB
4x4TB

Previously I had an 8 disk setup, which was all balanced, same disks minus 2 of the 4TBs.

So I added the 2 new 4TB drives, via the GUI, and it started a balance. Everything was going good until this morning. The GUI is showing the balance failed with the following message:

Error running a command. cmd = btrfs balance start -mconvert=raid10 -dconvert=raid10 /mnt2/MainPool. rc = 1. stdout = ['']. stderr = ["ERROR: error during balancing '/mnt2/MainPool': No space left on device", 'There may be more info in syslog - try dmesg | tail', '']

There is plenty of space, the overview tab shows:

Space free - 7.02 TB
Space used - 5.79 TB

And what is odd is that I got the out of space error when adding drives, which seems very strange.

At some point, it looks like a new balance was automatically initiated, not via the GUI. If I SSH in to the box, and do a btrfs balance status, it shows it as running.

So, I’m not exactly sure where I sit. I think my questions are:

  1. Why did the GUI balance say it ran out of space while adding disks?
  2. What does a failed balance mean for my data?
  3. If the second balance completes, should I assume everything is ok?
2

Could it be that BTRFS thinks I am out of space, when in reality I am not? Do I need to execute the command:

btrfs filesystem resize devid:amount /mount-point

Or does this GUI do this automatically when adding disks and rebalancing? Sorry, I am new to BTRFS, and just trying to understand how best to run things.