Originally Posted by jrl1357
How does a 7200 rpm drive have the same acces times as a 5400rpm one? This isnt sb vs bulldozer here, i think we can compear rpm. 5400 is less the 7200.. How does being 2.5inch affect it?
Access times aren't that dependant on RPM. Sure, it affects how fast it can go - but most drives are tuned in the firmware to seek conservatively. (AKA slow)
10000RPM = 166.67 RPS = 6ms per rotation.
7200RPM = 120 RPS = 8.33ms per rotation.
5400RPM = 90 RPS = 11.11ms per rotation.Western Digital:
Most 10k RPM Velociraptors have ~7ms access times.
Most 3.5" 7.2k RPM drives have 12 - 13.5ms access times.
Most 2.5" 7.2k RPM drives have ~15ms access times.
Most 3.5" 5.4k RPM drives have 15 - 17.5ms access times.
Most 2.5" 5.4k RPM drives have ~17.5ms access times.Hitachi:
Most 3.5" 7.2k RPM drives have 16 - 17.5ms access times.
Most 3.5" 5.4k RPM drives have 16 - 17.5ms access times.Seagate:
Most 3.5" 7.2k RPM drives have 13 - 14ms access times.
Most 2.5" 7.2k RPM drives have ~17ms access times.
Most 3.5" 5.9k RPM drives have 16 - 18ms access times.
Most 2.5" 5.4k RPM drives have ~17ms access times.
For every read or write there should be roughly a 50/50 chance that data will land in front of the head or behind it, so the average latency for the data to pass under the head should match the rotation time.
So why then can Velociraptors get within 1ms of their rotational latency, Blacks within 4ms, and yet 5400RPM drives are only within ~8ms of it?
Probably because the firmware is tuned conservatively. Remember that Green drives are supposed to be low power and quiet - they move the head where it needs to go much slower. It just mosies over to the right spot at its own pace, and if it takes an extra rotation, that's fine. Likewise, for laptop drives they don't want excessive vibration, so the head seeks are tuned to be slower and safer.
These conservative/lazy head movements skew access times higher. Any drive can be tuned that way, regardless of RPM. You can even force fast drives to have similarly high access times by enabling AAM for them.
Most people wouldn't understand that more aggressive head movements might be unsafe in a laptop, so companies use RPM as a marketing explanation. (same as Cache) Likewise, few people consider that WD Blacks have superior NCQ algorithms in the firmware, and may perform twice as fast as other 7200RPM drives at high queue depths. (It really shows when cold-starting Firefox) There's so many factors to a drive's performance, but most are unadvertised and locked away in the firmware. RPM and Cache are just the explanations they give us...Edited by Kramy - 4/5/12 at 7:09pm