It's TRIM with the HP controller. I have a similar problem, running it will completely fuck up my server, trying to get a process list via ps, or top will just hang, and eventually the server will just stop responding to just about everything. it won't reboot itself, and requires a manual reset via the BMC or by pushing the power button.
I managed to catch a kernel panic about "btrfs unable to handle kernel NULL pointer" the last time I ran it, but I was an idiot and forgot to save the full syslog output before I rebooted it.
As you can imagine, I'm not real keen on running it again