Many vendors sell 4GB SATA and Nearline SAS drives which spin at 7200RPM. The vendors will advertise a 6 Gbit/s interface with no mention of actual data transfer speed. I've had conversations with vendors, sysadmins, customers and technical managers who assume that 6 Gbit/s is the actual transfer speed of the disk.
Is it possible for a 7200RPM drive to fully utilize this 6 Gbit/s connection? If not, then does this inflated transfer speed offer any benefit to 7200RPM drive? Would a 6 Gbit/s interface offer any performance improvements compared to a 3 Gbit/s interface? Would this transfer speed be beneficial in a RAID array?
I've read that a typical 7200RPM drives will fully utilize the "buffer-to-computer" interface under ideal conditions. Performance features such as a large disk cache can only offer modest improvements to this speed.
Best Answer
A 7200RPM drive can load the cache and then burst the contents at the max bus rate, so yes, it is possible for a 7200RPM drive with a 128MB cache to fully utilize a SATA III link... for a whole 0.15 seconds. After that, sustained disk I/O will cause a bottleneck as the cache will need to be repopulated and you're waiting on the platters to catch up.
As mentioned in the comments, the SATA III (6Gbit/s) referenced in the advertising speaks to the interface, and not the data transfer rate. To my knowledge, the creators of the standard actually prefer people to call it "SATA III" because using "6Gbit/s" as the name is misleading as you have mentioned. Unfortunately, most people/vendors still use it.
Transfer rates for a RAID group of 7200RPM drives will certainly be better than that of a single drive, but the speed you actually get will depend on your hardware and RAID implementation.