Best option is to stripe pairs of mirrors. So in your case create a pool which 
stripes over 3 mirrors, this will look like:

pool
   mirror: 
      thumper1
      thumper2
   mirror: 
       thumper3
       thumper4
   mirror: 
        thumper5
        thumper6

So this will stripe over those 3 mirrors.

you can add mirrors if extra space is needed.

That's the way we implement it right now.

You can loose up to 3 servers (as long as they don't belong to the same mirror)

Of course the "nas head" is a single point of failure, and clustering iscsi 
zpools isn't that easy as you would hope :-( iscsi is not yet supported by the 
sun cluster framework. 

One of the drawbacks you can also expect is when you ever have to boot the 
nashead while some or all of the targets are unavailable, solaris is doing very 
nasty during start up, instead off just booting and putting the pool in 
degraded mode you will get the nashead hanging during boot untill you fix the 
targets, then the nashead continues to boot. 

If you really concern about speed I would advice you to use Infiniband and not 
ethernet, its also a good idea to Isolate the iscsi traffic.

K
 
 
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