Fix a stuck log device

ran into a troublesome ZFS bug several months ago where a pool with a log device became “stuck”. The ‘zpool remove’ command would complete but would not remove the device. This was a bad place to be in, because the device was no longer usable, could not be removed, and would most likely prevent the pool from ever being exported and reimported again. Someone else had posted on the zfs-discuss mailing list about the same problem and put me in contact with George Wilson, who in turn put me on the right track to a successful workaround. Log devices exhibiting this behavior need to have a specific field of a data structure zeroed out, and then the remove command will actually finish removing the device.

The procedure for fixing this is:

I experienced this problem on NexentaCore 3.1 which shares the same kernel bits with NexentaStor 3.x. Someone apparently filed a bug with Oracle (CR 7000154) but the information is unfortunately locked away behind their paywall. I haven’t looked recently but at the time there wasn’t an Illumos bug filed and it’s still not fixed. Hopefully the above information will help others experiencing this problem, however, please note that I am not responsible for any damage this might do to your data. Please make sure you have things backed up, and be aware that inputting anything to mdb in this manner can cause a kernel panic on a running system!