If you rip and put hdd back it zfs accepts it back to pool before you even get say pool.
In older raid systems if hdd is lost few second and came back raid would start rebuilding whole raid, and that takes long time, just because hdd had a little hiccup. zfs will handle hiccups better when hdd comes back from hiccup it just resilver latest stuff and not whole disk.
Edit:
Code:
zfs has many states:
ONLINE
The device or virtual device is in normal working order. Although some transient errors might still occur, the device is otherwise in working order.
DEGRADED
The virtual device has experienced a failure but can still function. This state is most common when a mirror or RAID-Z device has lost one or more constituent devices. The fault tolerance of the pool might be compromised, as a subsequent fault in another device might be unrecoverable.
FAULTED
The device or virtual device is completely inaccessible. This status typically indicates total failure of the device, such that ZFS is incapable of sending data to it or receiving data from it. If a top-level virtual device is in this state, then the pool is completely inaccessible.
OFFLINE
The device has been explicitly taken offline by the administrator.
UNAVAIL
The device or virtual device cannot be opened. In some cases, pools with UNAVAIL devices appear in
DEGRADED mode. If a top-level virtual device is UNAVAIL, then nothing in the pool can be accessed.
REMOVED
The device was physically removed while the system was running. Device removal detection is hardware-dependent and might not be supported on all platforms.
As you can see zfs can be come many states depend on hardware and a case.
if you rip it as in on it might be go REMOVED,UNAVAIL,OFFLINE,FAULTED,
The catch:
As for if you planning on just press offline button and rip off hdd and put new one while system is on, it might not work because your system might not be hotswappable and same time even not hotpluganable (hotplugin and hotswap is different things)
Thats why no one can say for sure does this online replace stuff works.
As for rip that hdd off onlinemachine you can try that if you want, no broblem but replacing new hdd while online what i have read its safer to shutdown, because some hardware might halt it.
(Yes you can take that hdd offline rip it or offline button or what pleases you most, but replace require reboot on nas anyway. And you put new hdd in and then you have to start nas and check does it show old or manynumbershdd offline state after that you manually press "replace this hdd" button) Sorry for my bad english.
Edit: now i see your point what you are after, manual says that you have offline disk before put new one on nas machine, and you thinkin what if hdd just actually goes bad while in on.
Just try see and or take it offline and then you get replace button (replace button seems changin time to time on different freenas versions)
I had one time that i couldn put that failed hdd offline, i dont have a clue why. But after rebooting few times nas while that old hdd was in place it then allowed to offline it, even it didnt see it anymore, after that i shutdown and put new hdd and continued from manual.
