Right now ztest executes zdb without -G, so when it has errors, the messages
are often not very helpful:
Executing zdb -bccsv -d -U /rpool/tmp/zpool.cache ztest
zdb: can't open 'ztest': Operation not supported
ztest: '/usr/sbin/amd64/zdb -bccsv -d -U /rpool/tmp/zpool.cache ztest' exit
code 1
With -G, we'd have:
/usr/sbin/amd64/zdb -bccsv -d -U /rpool/tmp/zpool.cache -G ztest
zdb: can't open 'ztest': Operation not supported

ZFS_DBGMSG(zdb):
spa_open_common: opening ztest
spa_load(ztest): LOADING
spa_load(ztest): FAILED: unable to parse config [error=48]
spa_load(ztest): UNLOADING
Which indicates where the error came from