I updated the signature. I explained it above. It's only 9TB usable.
Okay, that makes more sense. I figured either I was reading something wrong or you'd accidentally written the raw space before parity.
And yes, you definitely want to avoid SMR in the future. There's a (partial) list and a statement from iXsystems here:
Thanks to the FreeNAS community, we have uncovered a potential ZFS compatibility issue with some capacities of newer WD Red drives that use SMR (Shingled Magnetic Recording) technology. Update to this post...
www.ixsystems.com
A potentially relevant part is bullet point 4:
At least one of the WD Red DM-SMR models (the 4TB WD40EFAX with firmware rev 82.00A82) does have a ZFS compatibility issue which can cause it to enter a faulty state under heavy write loads, including resilvering. This was confirmed in our labs this week during testing, causing this drive model to be disqualified from our products. We expect that the other WD Red DM-SMR drives with the same firmware will have the same issue, but testing is still ongoing to validate that assumption.
If your SMR drive had a hiccup and went into a "took too long to respond" sort of state, that could have caused ZFS to kick it out momentarily, and then when it reappeared it resilvered the data and everything was good again. But in your case I'd pull a
smartctl -a
of all drives and take a look for any signs of early failure (eg: non-zero values in
Reallocated_Sector_Ct,
Current_Pending_Sector, or
Offline_Uncorrectable) - if it was one of your
other drives that choked up, you might need to buy two new (non-SMR) drives to get yourself back to full health and full speed.