Hard Drives Explained – A Supplement to BYOB Podcast #24

Serial Advanced Technology Attachment Spec

Three speed specs for SATA – They are confusing with the numbers, so here you go:

• Version 1.0: SATA 1.5Gb/s – Initial interface speed for 150MB/s data transfer rate
• Version 2.0: SATA 3Gb/s – Enhanced interface speed for 300MB/s data transfer rate, backward compatible
• Version 3.0: SATA 6Gb/s – Most recent interface speed increase providing 600MB/s data transfers and backward compatibility

Want to fully understand the above I/O (input/output) SATA specs?

Here is another math problem: The Version 3.0 spec is 6 Gigabits. That is 6,000,000 bits, but divide it by 8 to get 750 MB (750,000). But you can only get 80% due to 8b/10b encoding, so you end up with 750 MB x .80 = 600MB which is the spec that manufacturers put on their pages.

Disk Buffer

Per the Western Digital Knowledge Base “Cache memory is the data buffer or cache between the hard drive and the actual platters in the drive where data is temporarily stored. Access to data in the memory cache is much faster than accessing data on the platters in the hard drive. The larger the memory cache, the more data can be stored which can be accessed faster.”

What are the benefits between 32MB, 16MB, 8MB, or 2MB cache drives-

In preparing this sermon I had to search to really find any performance data that relates to the disk buffer. The performance distinction between disks that have over 8MB is not necessarily a determining factor when buying a drive. Most buffers were at 2MB in 2001 and have worked their way up to 64MB for the latest high-end drives available. Even most consumer drives now come with 16MB or more so the performance comparison it is not really an issue.

You probably will not notice a performance difference with disk buffer amounts. Again, I could not find any reliable data for buffer performance and will just reference the WD quote above.

Power Consumption and Sound

Power is rated in watts. Green drives run fewer watts. Black Caviar and Seagate XTs run more watts. There is also a relationship to watts and heat, so more watts usually means more heat.

If power is important then do the math (watts x 8.76 x kWh cost) for a yearly cost. For example, the 9 watt Seagate Barracuda XT at 15 cents a kWh will cost me $11.82 per year. It would cost me half that in Canada, right Diehard?

Watts it all about?

Sound could also be a factor. More RPMs means more sound. In addition, desktop drives usually make more noise than laptop drives.

Sounds good to me?

For a great list of Hard Drive sound reduction products, check out FrozenCPU!

Reliability

MTBF is often used to gauge drive reliability. A vendor-quoted MTBF implies that half the drives in a large population will fail within that time of operation. This does not mean that you will get those hours out of your drive.

Another way to check a drive manufacturer’s reliability is with the Annualized Failure Rate. This rate is the relation between the mean time between failure (MTBF) and the hours that a number of devices are run per year, expressed in percent.

Both ways can be irrelevant to your situation or system so just backup your data!

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Conclusion – Overall Drive Performance

Get the best drive you can for the money. Find a brand you like and do the research. Check the specs.

For example:

• Using sustained transfer rates for comparison is good if you are going to use your drive for massive storage and file transfers but may not be indicative of your actual performance
• Latency figures should all be about the same for each RPMs category so watch them
• Random read and seek times are a great way to see how daily usage will be affected
• I/O data transfer times are theoretical and nowhere near realistic – Overhead
• If power is important then compare watt ratings
• If sound is important the check the dB ratings

And there you have it.

Timothy Daleo

References

http://www.sata-io.org/technology/6Gbdetails.asp

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