Ubuntu hangs at startup due to zfs

Question

I installed zfs-on-linux to my virtual Ubuntu LTS server years ago via PPA. The system resides on a VMware Workstation 10 in Windows 8.1.

The working ZFS/SPL versions installed are 0.6.3. It has endured a few OS upgrades from earlier LTS's to 14.0.2 LTS. My zfs pools host /home, /var/www, and other data directories that shouldn't be part of the boot process. I recently cleared some unused kernels and kernel sources with apt-get autoremove. At reboot, the system hangs at:

Scrolling back, I see some potential errors:

[   17.287100] EXT4-fs (sda1): INFO: recovery required on readonly filesystem
[   17.287190] EXT4-fs (sda1): write access will be enabled during recovery
[   17.297142] EXT4-fs (sda1): recovery complete
[   17.297810] EXT4-fs (sda1): mounted filesystem with ordered data mode. Opts: (null)

If I reboot to recovery mode, do zfs mount -a, and resume booting, everything works fine, until I need to reboot again. One strange behavior in the recovery mode is that zfs list (after zfs mount -a) won't show the datasets as mounted even though they are mounted. I think it's because the root filesystem (on /dev/sda1) is still in ro mode.

I also tried:

• fsck /dev/sda1 -- It reports the FS is clean, and this step does not solve the problem.
• Completely purge zfs from system with dpkg --purge using this guide . In this case the system can reboot to login prompt (There's no GUI) without problem, but I can't use zfs. As soon as I re-install ubuntu-zfs and reboot, the system gets stuck at the same spot.
• Install another version of Linux kernel and headers. I've tried 3.16.0-31-generic. My other working systems with zfs have 3.13.0-35-generic and 3.13.0-44-generic, so I also tried them. Result: Same problem.

Any ideas on this problem or how to continue the investigation?