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> This is because RAID-Z spreads each block out over all disks, whereas RAID5
> (as it is typically configured) puts each block on only one disk.  So to
> read a block from RAID-Z, all data disks must be involved, vs. for RAID5
> only one disk needs to have its head moved.
>
> For other workloads (especially streaming reads/writes), there is no
> fundamental difference, though of course implementation quality may vary.
streaming workload generally is always good. random I/O is what is 
important.