Can a 7200RPM drive transfer speed at a full 6 Gbit/s?

Question

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.

Solution 1

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.

Solution 2

you need to check the hard drive's spec sheet. they will show average seek time, average write/read transfer rates in MB/sec every drive manufacturer has this info on their site. Striped RAIDs are at least as fast as a single drive, meaning their theoretical slowest speed is still the speed of one single drive. It all has to do with how hard drive data is being accessed

Comments

[jlehtinen]

Are you sure they are actually advertising 6 Gbit/s transfer rate, and not just saying they have a SATA III (6Gbit/s) interface?

[jlehtinen]

In the data sheet for the product you linked, it states Max Sustained Transfer rate is 175MB/s...

[vanowm]

@jlehtinen exactly, so why do they shlap more expensive 6gbps or even 12gbps interface on hard drives that can't possibly produce that much transfer rate?

[vanowm]

So, other words they do that to mislead? There are SAS hard drives with 12gbps interface speed that can only transfer at 250Mb/s (aka 2mbps). I can understand interface speed on controllers, which makes them future proof, but on hard drives - it just doesn't make sense, even for "0.15 second".

[smammy]

@jlehtinen, it turns out that SATA-IO prefers "SATA 6Gb/s" and specifically calls out "SATA III" as incorrect. To wit: IMPORTANT: Do not use the terms "SATA III" or "SATA 3.0," which will cause confusion. For product naming purposes, do not use either "Third Generation" or "Gen3" in the product name. Use "SATA 6Gb/s" instead. Please note that the references embedded within the new specification to "Gen3" are technical specification naming conventions only and should not be used for marketing and product naming purposes. See: sata-io.org/developers/sata-naming-guidelines