Ask the Expert

SCSI/FC disks vs. IDE/ATA disks

I read your Ask the Expert Q&A where you stated that:

"SCSI and Fibre Channel disks are manufactured to much stricter tolerances than IDE disk and even ATA disks. The "duty cycle" for IDE and ATA is "spec'ed" much lower than that of Fibre or SCSI disks. This means the bearings and the actuator arm mechanics used in the higher priced drives are rated for more useful life than both IDE and ATA. SCSI and Fibre disks are built to get hammered by read/write data requests all day long, every day, for years. IDE is built to sit in a PC or laptop and get spun down when you go home."

Can you please tell me where can I find a document and information that can prove those differences between IDE and SCSI HDD especially the fact that "bearings and the actuator arm mechanics used in the higher priced drives are rated for more useful life than both IDE and ATA"?

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All you need to do is go to any disk manufacturers Web site and pull down the spec sheets for the individual drives. You can try HGST if you like which will give you information on IBM and Hitachi drives.

One of the important factors you will notice within the spec sheets are the MTB (mean time between failure) ratings for each type of drive. You will find that the ATA type drives, which are built to be placed in PC type environments and therefore priced cheaply, will be rated lower than those built for very high duty cycle environments like RAID arrays. If you look closely at the spec sheets, you will notice different components being used within the drives built for different environments.

The high-end, 2.5 million hour MTBF drives used in high duty cycle environments will cave more exotic components, like "fluid dynamic bearing spindle motors", "acoustic and shock dampers" and AFC media (AntiFerromagnetically Coupled) based on "glass substrate."

The lower cost drives like IDE, ATA, etc. will most likely use less exotic (and less expensive) components and have lower ratings for vibration, temperature, shock and acoustics. You will see MTBF ratings in the 300,000 to 500,000 hour ranges, and lower duty cycles (say 20% rather than 100%). Also look for variations in POH (power on hours) and start/stop cycles. It all comes down to the old adage "you get what you pay for."

Hope this helps.

Chris

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This was first published in January 2004

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