Boot mirror "use all disk space" feature

[sleeper52]

Newb question here. I'm planning in creating a boot mirror on a 2nd flash drive for my FreeNAS OS and I see a "use all disk space" checkbox option.
What would be the pros/cons of having this ticked/unticked? What would be a usecase in which I would need to use this feature? Are there performance gains?

 

[dlavigne]

The Guide says this:

The Use all disk space option gives control of how much of the new device is made available to ZFS. The new device is partitioned to the same size as the existing device by default. Select Use all disk space to use all available space on the new device. If either device in the mirror fails, it can be replaced with another of the same size as the original boot device.

When Use all disk space is enabled, the entire capacity of the new device is used. If the original boot device fails and is removed, the boot mirror will consist of just the newer drive, and will grow to whatever capacity it provides. However, new devices added to this mirror must now be as large as the new capacity.

Does that answer your concern?

 

[sleeper52]

The Guide says this:

The Use all disk space option gives control of how much of the new device is made available to ZFS. The new device is partitioned to the same size as the existing device by default. Select Use all disk space to use all available space on the new device. If either device in the mirror fails, it can be replaced with another of the same size as the original boot device.

When Use all disk space is enabled, the entire capacity of the new device is used. If the original boot device fails and is removed, the boot mirror will consist of just the newer drive, and will grow to whatever capacity it provides. However, new devices added to this mirror must now be as large as the new capacity.

Does that answer your concern?

I actually read this on the docs but it doesn't really answer my question. No big deal. I just left this unticked. As far as I can tell there is no real benefit to this and I fail to see the necessity of adding this 'feature.'