Running Kubuntu Vivid (15.04) using a BTRFS filesystem and a more recent than out of the box 3.19.0-22-generic kernel. I have a couple of times experienced an apparent lock up on boot with only "starting version 219" in the top corner of the screen. Strangely, I would boot from USB, mount the drive, reboot, and it worked again.

I suspect this is a different issue. Everything was fine. I installed a couple of packages, shutdown, later on, started up and the display and desktop were broken. Displays were mirrored instead of the desktop spanning the two, the desktop could not be seen. Yakuake drop down terminal started and dropped down as normal, but filled the entire screen. The mouse cursor, starting before switching from lightdm to KDE, disappears, and only shows up in the windows "yakuake has successfully started" or in the yakuake window once it appears. A pretty bad failure.

For just this sort of thing, I have apt-btrfs-snapshot installed and working. But I rolled back to several previous snapshots, back to 2 weeks ago when I know for sure it was working perfectly, and I get exactly the same problem.

Then I scrub the hard drive to see if there are any checksum errors. None. Not one on the whole drive, so I am at a loss.

If everything is working as it should, and the checksums match the data on the drive, then a rollback to a working state should work, 100%, no question. It couldn't be bit rot or some sort of physical corruption because the checksum wouldn't match. I'm no computer scientist, but the only logical explanation I can think of is some sort of problem or bug with the BTRFS file system, which, although relatively new and undergoing rapid development is supposed to be stable (yes, use it at my own risk, I do have a backup). It would have to affect previous snapshots, which could easily be data that has not changed and is therefore common to all snapshots, but would have to escape checksum verification.

This is my OS snapshot, but what if similar things are happening to my home directory? I could be scrubbing, and running incremental backups, thinking it's fine, but there are errors BTRFS is failing to tell me about.

Any idea what could have caused this?
Is there any way to avoid it?
Or is this the kind of risk one takes, however unexpected, when using btrfs?

If I saw those symptoms, I'd be suspecting hardware not software straight away. You could start with the easy stuff - check cables, give everything a good blow-out - then I'd be looking at PSU.

I've used btrfs for many years - snaps being a major attraction. Never seen anything like this - not saying it's not possible. I might be inclined to send the filesystem to another system just to satisfy myself it is ok, but I'd have pretty high confidence in it.

Thanks, I'll do that and see how it goes. It's good to hear you've never had such a problem with btrfs at least.