Here's where I tell you "OMFG".
Because you are running a virtual machine you are instantly out of the scope of my ability to help. In fact, if you had mentioned you were using a virtual machine for FreeNAS before you would have gotten about 5 people that would have responded with "good F'in luck getting your data back". I probably wouldn't have even responded if you had said you had used virtualization. It's almost like the dummy's penalty for not following the recommendations for use of ZFS. It's not that people don't want to help you, its that all options for recovery go away the second someone uses virtualization. Any kind of troubleshooting needs to be done on the bare metal and then the recovery actions need to be done on the bare metal considering the actual configuration of your virtual machines, then adjusted accordingly to take into account your configuration and your virtual machines, then applied properly inside the virtual machine itself. That's something that can't be done reasonably over a forum or a remote session. It's something only the administrator will know, and if it wasn't well planned ahead of time there may not be a recovery option.
My guess.. the real disk that has the virtual disk ada2 on it is failing, has failed, or the virtualization is failing, or has failed, or you have a software conflict in the hypervisor itself causing the problem. The possible causes are almost endless because of the significantly more complex layer of virtualization. Because you virtualized FreeNAS and ZFS(which is a major no-no) ZFS is confused as all hell because it doesn't have enough information to realize that ada2 is broken.
At this point the best idea I can come up with is to figure out which physical disk "ada2" is actually stored on and run some SMART tests. I'd wager the disk is failed, but FreeNAS has no way of identifying and/or proving it because you virtualized it. If the physical disk "ada2" is on is bad you can try a recovery using something like ddrescue in FreeBSD or Linux on a different machine to copy the contents of the bad disk to a spare good one. With some luck you might be able to get some of your data back. But good luck in any case because I don't have any other ideas. There's a reason why there's threads titled things like
Please do not run FreeNAS in production as a Virtual Machine! and then threads like
"Absolutely must virtualize FreeNAS!" ... a guide to not completely losing your data. The last link posted says "I am not aware of specific issues that would prevent Xen from being suitable. There is some debate as to the suitability of KVM. You are in uncharted waters if you use these products." Both of those threads are the product of more than a dozen people losing everything because they thought they were so smart by virtualizing FreeNAS and ZFS. Unfortunately, I can't even trust that the I/O error message you are getting is truely an I/O error because you are virtualizing. Basically every error at this point in the virtual machine is not necessarily what it seems.
ZFS really truly needs bare metal. Virtualizing removes a lot of the technological solutions ZFS is supposed to solve for redundancy and adds a lot of complexity that the server administrator is responsible for planning for, troubleshooting, and resolving on his own.
Unfortunately, you are truly on your own to fix it :( Good luck. I'd keep a record of what you do and what you find. If you do manage to get your data back it might be helpful to post what you did. I've never used or seen anything except VMWare products. Personally, I don't think I've seen anyone use virtualization and actually recover their data. :/ There is one forum member that may have some advice, but I believe his advice will be "recover for backup or give up". I also won't give his name because he's so incredibly tired of people not being smart with Virtualization he doesn't generally even help people anymore. It's really something he's had to deal with weekly(sometimes twice a week) for months. Yes, that many people end up losing their data because of virtualizing.
Edit: The one person with alot of virtualizing experience, if he chooses to show himself, will probably post or PM you in the next 24-48 hours. If not, then you are on your own. You may want to try to run diagnostics on your hard drives(nothing destructive of course) just in case he decides to help you. Naturally those hard drive diagnostics may at least rule out a hard drive failure even if you can't get your data back.
