Performance gain when adding drives to Raid 5?

I am looking for any benchmark data regarding i/o performance gains when adding more drives to a raid 5 array. I see lots of comments that performance will increase, but looking for some idea of what to expect , ie if I double the drives will I double the i/o (assuming no bottlenecks elsewhere).
medentAsked:
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zdesCommented:
Not necesserily. Your controller can get saturated. Then you get less than two-fold. Or you already have sufficient performance, and you will get no gain, because your system is not disk I/O bound. Or you keep the same amount of data, but spread it evently across two RAID 5 devices so that the data ends up being on more dense tracks. Then you can get more than 2-fold gain.

There was a nice book about general performance configuration by SunPress (Yeah, I know that it is a linux question :-)  Brian Wong "Configuration and Capacity Planning for Solaris Servers". Good read. It will also help you to know exactly what you are looking for.

If you are after performance, and ready to double disks, RAID-5 is not the best option - striping with mirroring would be better.
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