Feeds

Disk platter sizes vary with drive speeds

Faster speeds equal small platters

HP ProLiant Gen8: Integrated lifecycle automation

Comment When is a 2.5-inch form factor drive not a 2.5-inch form factor drive?

A small form factor (SFF) drive comes in an industry standard 2.5-inch form factor case size. Inside that there can be one or two platters and which can be smaller or larger. Yes, they will be smaller platters than ones found in 3.5-inch form factor cases, but the largest of them can nearly equal a small platter found inside a high-speed 3.5-inch form factor disk drive.

The thing is that rotating a platter at high speeds is a hard thing to do because the higher the speed the stronger, and generally larger, the platter central mounting has to be, and the physically smaller the platter has to be so that the outer areas don't wobble and cause a data mis-read or worse, as in head crash worse.

What this speed:size relationship means is that a Seagate Momentus 7200.9 SFF laptop drive spins at 7900rpm and has a larger platter size than a Savvio SFF drive spinning at 15,000rpm.

This means that areal density comments about drives have to be made with reference both to the external case dimensions - 3.5 or 2.5-inch - and to the internal platter size. Generally such areal density statements don't refer to the platter size at all. A SFF Momentus 7900 might have 250MB/platter while the SFF Savvio 15K.2 has 148MB, but that doesn't mean the Savvio has lesser areal density. It has a smaller physical platter area and that's why its capacity is so much less.

In the enterprise storage area, a 3.5-inch Barracuda ES.2, spinning at 7,200rpm, can hold a maximum of 1TB on its two platters whilst a Cheetah 15K.8, spinning at 15,000rpm, can hold only 450GB on its two smaller platters. But then it is a much faster drive.

There are no generally known industry standards for platter size dimensions. We can't assume that Hitachi GST, WD, Toshiba and Seagate SFF 7900rpm drives all have the same platter dimensions, for example. Ditto at the 3.5-inch level. We can be reasonably certain, though, that at the largest capacity sizes in any form factor and low-to mid-speed rotation the largest possible platters are put inside the cases.

So remember, when talking areal density your mileage may well vary and you have to be making apple-for-apples comparisons, which you won't know unless you're aware of the platter sizes involved. ®

Eight steps to building an HP BladeSystem

More from The Register

next story
THUD! WD plonks down SIX TERABYTE 'consumer NAS' fatboy
Now that's a LOT of porn or pirated movies. Or, you know, other consumer stuff
EU's top data cops to meet Google, Microsoft et al over 'right to be forgotten'
Plan to hammer out 'coherent' guidelines. Good luck chaps!
US judge: YES, cops or feds so can slurp an ENTIRE Gmail account
Crooks don't have folders labelled 'drug records', opines NY beak
Manic malware Mayhem spreads through Linux, FreeBSD web servers
And how Google could cripple infection rate in a second
FLAPE – the next BIG THING in storage
Find cold data with flash, transmit it from tape
Seagate chances ARM with NAS boxes for the SOHO crowd
There's an Atom-powered offering, too
prev story

Whitepapers

Seven Steps to Software Security
Seven practical steps you can begin to take today to secure your applications and prevent the damages a successful cyber-attack can cause.
Consolidation: The Foundation for IT Business Transformation
In this whitepaper learn how effective consolidation of IT and business resources can enable multiple, meaningful business benefits.
Designing a Defense for Mobile Applications
Learn about the various considerations for defending mobile applications - from the application architecture itself to the myriad testing technologies.
Build a business case: developing custom apps
Learn how to maximize the value of custom applications by accelerating and simplifying their development.
Consolidation: the foundation for IT and business transformation
In this whitepaper learn how effective consolidation of IT and business resources can enable multiple, meaningful business benefits.