Basically I think yes you need to add all the vdevs you require in the
circumstances you describe.
You just have to consider what ZFS is able to do with the disks that
you give it. If you have 4x mirrors to start with then all writes will
be spread across all disks and you will get nice performance using all
8 spindles/disks. If you fill all of these up then add one other
mirror then its logical that new data written will be only written to
the free space on the new mirror and you will get the performance of
writing data to a single mirrored vdev.
To handle this you would either have to add sufficient new devices to
give you your required performance. Or if there is a fair amount of
data turn around on your pool, ie you are deleting (including from
snapshots) old data then you might get reasonable performance by
adding a new mirror at some point before your existing pool is
completely full. Ie data will initially get written and spread across
all disks as there will be free space on all disks, and over time old
data will be removed from the other older vdevs. Which would result in
most of the time reads and writes benefiting from all vdevs, but it't
not going to give you guarantees of that I guess...
Anyway, thats what occurred to me on the subject! ;)
cheers Andy.
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