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I have a 3ware 8 channel SAS controller and a back plane extender (also 8 channel) which can take 16 drives. I will be creating a single RAID 10 volume.

I know that adding more drives has positive effect on performance, but I'm not sure if adding more than 8 drives on an 8 channel controller will have any positive impact at all. Am I wrong?

Should I put 16 drives for best performance? Would 8 drives give me the same performance?

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What model 3ware RAID controller is it? – Mark Davidson Sep 3 '09 at 12:55
It is 9690SA-8I with 512MB cache. – Tvrtko Sep 3 '09 at 13:11

My basic gut reaction would be to fill the controller if you can. If I understand correctly, it mirrors on the channel and stripes across them, suggesting that 16 drives (2 per channel) would be the smart thing to do.

I may be wrong, however.

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The SATAII spec...also called SATA 300 and SATA 3Gb just means that the bus rate is 3Gbit/s which works out to be roughly 300MB/s. Hence the other names. However a single drive doesn't saturate this bus. Not even the SSD drives can saturate it currently.

Your controller has two multi-lane 4x ports. As long as the extender also has 2 multi-lane 4x ports then you should be able to safely put 16 drives without introducing any bottle necks. At least not between the drives and the controller.

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