[Bug 245186] zfs panic despite failmode=continue

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=245186

--- Comment #2 from John F. Carr <jfc at mit.edu> ---
I understand it's a different path internally, but I asked for disk errors not
to crash the system and that's what I expect to happen.

The code in spa_misc.c appears to allow 1,000 seconds.  I've seen sync take a
significant fraction of that time with working disks.  I/O on a failing disk
can be orders of magnitude slower than usual.  It might take seems like forever
to work through the queue, but the driver is continuing to process I/O
requests.

Unfortunately based on comments the deadman timer is based on the oldest
pending I/O.  If the kernel used a per-disk timer that counted time with a
non-empty queue and no requests completing it would be able to distinguish a
very slow disk from a hung driver.  Or it could maintain some counter of failed
I/O and mark the disk dead when the rate got too high.

I think the drive should be kicked out of the pool and its I/O queue flushed in
this situation.  When my drive first started failing that's what happened.  I'd
run zpool status and find one of the drives removed.  I could run geli attach
and a zpool command to bring it back in until the next time it got kicked out. 
More recently the system started crashing instead.

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