Yes, it is free space fragmentation, but that's no excuse for the system to lock up.
You might want to review your storage setup, but that's a different story.
Uh oh, lol. Mind sharing what I am doing wrong? just a quick side bar, not to deviate from the main topic to much. But if you see something wrong, I'll be sure to fix it, Plex and Emby sit on those 2 mirrored SSDs, and they are pretty active IO wise as they manager a decent sized library, and they have been running for 2 years on the same mirror pool.
SuperMicro SC826E16-R1200LPB Chassis 2U | 12 Bay
SuperMicro X8DTN+ | 2x Intel Xeon Six Core E5645 @ 2.4Ghz | 48GB DDR3 ECC
BPN-SAS2-826EL1 826 backplane with single LSI SAS2X28 expander chip
Dual 1200watt PSU
LSI 9240-8i Controller
FreeNas Boot
2x SanDisk 16GB Cruzer Blade SDCZ50
RAIDZ2
6x Seagate Enterprise 5TB 7200RPM SATA III ST5000NM0084
RAIDZ1
4x Seagate SV35 Series 3TB 7200RPM SATA III ST3000VX000
Jail Mirror
SanDisk 128GB SSD SATA III Z400s (soon to be replaced with another evo850)
Samsung EVO 850 250GB SSD SATA III (brand new drive that just resilvered, replaced the other SanDisk 128)