ZFS failure leads to panic/boot loop

[Michael Wayne]

11.0 server installed from initial setup with ZFS mirrored drives. 

Ran fine for a few weeks sitting in the rack, then became unresponsive.

Plugging in a console showed it was in a panic / boot loop due to ZFS.

Would like suggestions on what is wrong that caused this problem
as well as how to proceed to recover from it.

Typing this from a phone photo of the console screen so showing
only what I believe to be relevant; I can supply actual addresses
if required.  Boot gets to:

[...]
uhub2: 4 ports...
panic: Solaris(panic) blkptr at 0xfffffe0003244b80 has invalid CHECKSUM 0
cpuid - 0
KDB: stack backtrace
[...]
   zfs_panic_recover
   zfs_blkptr_verify
   zio_read
   spa_load_verify