JJ Johnson

Do SATA III controllers offer any advantage over SATA II?

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If you use a drive cage with port multiplier, it can be worth a SATA III controller. Individual hard drives can exceed 200 MB/s today at the beginning of the drive. With as little as two drives in your cage, you can saturate a SATA II link. Say if you're copying a file between the drives.

If you have 4-5 drives RAIDed in your port multiplier enclosure, you could see a big boost from a SATA III controller. However, your PM enclosure must be SATA III also.

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In rare cases where you miss (or disable) OS-level disk cache, but get a cache hit in the drive, the read will complete faster. It's hard to see how that would have a noticeable impact for the end user, except in some rare corner cases and specific circumstances.

In common usage, the only difference would be a slight reduction in read/write latency. However, write latencies are usually hidden by the OS and read latencies are usually dominated by rotation and/or seeking. Perhaps it would be possible to measure this on a fast 10k drive, but it should be fairly negligible.

Now for some hypothetical stuff with lots of cavaeats. I'm not entirely sure what's the maximum SATA packet payload, but some ACHI docs I found seem to imply it's 4 MB. Now, if a drive reassembled this chunk before starting to send it to the controller (again, I don't know if SATA works like this), you could be talking about 13.3 ms to send it via SATA 2 vs. 6.7 ms to send it via SATA 3. But if drives really worked like that, then controllers or the OS would probably break such large reads into smaller ones, so that the first parts of the read could be transmitted while the drive accessed the later parts.

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