RPM vs Transfer rate?

I built my current computer, but when I did, I really didn't focus on HDD specs. I got a WD1500ADFD Raptor to run my OS and programs and a WD500AAKS Caviar Blue to store project files and such. I am getting close to updating my workstation, but this time around I want to be a bit more educated.

I noticed my Raptor's specs are: 10,000rpm / 1.5 Gb transfer rate / 16MB cache
My Caviar Blue's specs are: 7200rpm / 3 Gb transfer rate / 16MB cache

So my question is - The Raptor is of course faster, but how important is transfers rate compared to speed as it relates to a HDD's overall performance?

Also, I saw this in my Caviar Blue's spec sheet:

Transfer Rates
Buffer To Host (Serial ATA) 3 Gb/s (Max)
Transfer Rate (Buffer To Disk) 972 Mb/s (Max)

What's the difference between the Buffer to host and Buffer to disk? Should I go by one over the other?

Thanks in advance!
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More about transfer rate
  1. The original SATA design could transfer data from the HDD (actually, from its buffer RAM) to your system at 1.5 Gb/s max. Same max speed the opposite direction. Then SATA II increased that max speed to 3.0 Gb/s. BUT the speed of moving data between the HDD's buffer RAM and the actual disk platters is much slower. It depends on several things, but disk rotational speed, or rpm, is a key part of this. The net result is that for MOST hard drives and MOST types of data transfer jobs, the long-term AVERAGE data transfer rate is LESS than the 1.5 Gb/s maximum burst speed of the original SATA system. NOTE that this generality is NOT true of the new SSD units since they do NOT depend on physical rotation of platters to bring data to heads for reading.

    In the current market, you will not find any new HDD running at the old SATA data transfer speeds - everything is SATA II, or the new 6 Gb/s version of SATA. HOWEVER, for actual hard drives with spinning platters for data (that is, not SSD's), NO HDD can moved data between disk platters and buffers faster than the SATA II spec, so there is no real need to go for a "SATA III" - more properly, SATA 6 Gb/s - HDD.

    Now on rpm's there certainly is an advantage to higher rotational speeds, but it costs a bunch of money. WD's Blue line tends to be slower than the Black line, and the Raptor line is faster yet. Between Blue and Black is their Green line that emphasizes low power consumption and is a little slower than the Blacks. Yet another factor is the size of the buffer in the HDD. Many now are 32 MB (the Blues are less), and some among the Green line go to 64 MB. Higher buffer size is an advantage for certain types of data file access, so putting more on a Green drive with a slightly slower rotational speed is a way to boost its performance to compensate.

    Search out on Tom's some of the lab tests of disk performance. They will explain the various types of data file access patterns involved and which HDD properties are more important for each. This will help you decide, on the basis of your typical uses, what is more important for you. And you'll gain a better understanding of the specs, what they mean, and how to separate the hype from the facts.
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