Adding a second device to my zfs pool

[Michal]

I am having problems getting my head around a small ZFS issue. If I
build a fileserver, say for example a 20 x 1TB disk server with zfs and
create a file server, it can work fine and I can have RADIZ or RAIDZ2 or
whatever. I could create 4 groups of 5 1TB HDD's and do all things like
that...It's shared using iSCSI or samba or whatever, desktops and
servers can see the files, everything works fine. But one day I run out
of space, so I think right, build an exact copy of the one I have so I
have another 20 TB server, given me 40TB, but I want nothing to change
for the desktops/servers and as far as they know nothing as happened.

But my problem is I could set that second server up, but how do I add
the space to my existing pools? I create my RAIDZ2 zpools of hard
drives, the raid works and everything, but now I want to increase my
existing storage. I could go to my first server and simply add the
storage to my groups, thus increasing the size...but then if that server
fails I will lose all data? will I not?

Basically I'm having trouble finding information on what happens when
you add a second file server to increase your space. I see lots of
guides and documents for 1 device and adding more HDD's to that device,
but not when you need to add a second device (run out of HDD space in
the device for example)

Thanks