I recently installed OMV 5.x on my NAS (from Ubuntu 18.04) utilizing an existing mdadm RAID1 config. One of these RAID disks was starting to fail (high amount of unrecoverable sectors). I decided to buy 3 x 3TB (Ironwolf) drives and configured two into a mergerfs pool (with a most free space create policy) and enabling SNAPRAID with a single 3TB parity drive. All went well with this configuration. I copied all my data off the RAID array onto the mergerfs pool without issue. I then decided to remove the now empty RAID array, take the one healthy disk (reformat) with a new file system and add to the mergerfs pool.
Now the problem. When I write files to the pool, the new disk is not receiving any blocks. I keep on having to run the tool mergerfs.balance to resync the data. This works to within 2% of free space (default with the balance tool), but the problem still persists. I've rebooted the NAS thinking that a re-mount of the pool is required but this has not been effective either.
Any suggestions on how to troubleshoot? The drive in question is DATA2. Right now there is 5% difference but as you can see with DATA3 and DATA4 they are closely matched.
/dev/sda1 1.8T 507G 1.3T 28% /srv/dev-disk-by-label-DATA2
/dev/sdc1 2.7T 893G 1.9T 33% /srv/dev-disk-by-label-DATA3
/dev/sdg1 2.7T 892G 1.9T 33% /srv/dev-disk-by-label-DATA4
/dev/sdf1 2.7T 811G 1.9T 30% /srv/dev-disk-by-label-PARITY1
MEDIA_UNION:eb06833a-73ce-437e-87c7-d3c2c5c2ae8b 7.2T 2.3T 5.0T 32% /srv/eb06833a-73ce-437e-87c7-d3c2c5c2ae8b