Critical Section

hard drives: faster isn't necessarily faster

Tuesday,  01/15/08  09:13 PM

Okay, so here’s some esoterica for you… on the subject of laptop hard drives.   Please feel free to skip…

Windows being what it is – a horribly inefficient OS which has poor paging algorithms – disk performance tends to be just as important as CPU performance in the overall “feel” of a system.   Historically this is a key reason why laptops have felt slower than desktops; the CPUs are nearly as fast (even with their power management algorithms), but the 2.5” disks found in laptops have been way slower than their 3.5” desktop brethren.

Since carrying around a desktop is a bit awkward, finding a fast hard drive for your laptop is a good thing to do.

So there are four characteristics of a drive which determine its performance.   First you have seek time, the speed with which the disk heads move across the disk to the desired track.   Second you have rotational speed, which affects the latency of waiting for the desired sector to spin under the heads.   Third you have transfer speed, the speed at which bits are moved from the drive to main memory.   And fourth you have the cache size; a larger cache allows the disk to do more reads faster, because read B can be started while read A is being transferred (and for a big cache, read C and D and … can be started too).   These four factors are important in roughly the order I listed them above.

A couple of years ago I began with the “stock” drive that came in my HP nc8230 laptop.   (for longtime readers, yes, that laptop :)  It was a 60GB hard drive, spinning at 5400 RPM, with a 2MB cache.   About a year ago I upgraded to a 100GB hard drive, spinning at 7200 RPM, with a 4MB cache.   Big difference!   In addition to having 40GB more for storing digital slides (!), the drive was noticeably faster, and the whole laptop felt faster.   A good thing.

Recently I began poking around for a new drive, since I had managed to fill my entire 100GB with files.   Plus I thought maybe drives had become faster in the interim.   So what I found is that the largest drive available spinning at 7200 RPM is still about 100GB.   In order to get a bigger drive, I’d have to get a “slower” drive spinning at only 5400 RPM.   How much slower would this drive be?   Not clear.   So in the name of science I ordered a 250GB drive, spinning at 5400 RPM, with an 8MB cache.

Guess what?   It is faster.   Noticeably faster, the whole laptop feels faster.   Which is a good thing.   But, um, it is only 5400 RPM, not 7200 RPM.   What is going on?   Well it turns out there are four factors in hard drive performance :)   And the seek time of this drive is considerably faster than the seek time of the “faster” 7200 RPM drive.   (As a general rule, the larger the capacity of a drive, the faster it seeks, because to access any given amount of data requires less head movement.)   Furthermore the transfer speed is faster (slightly), and the cache is twice as large, allowing more activity to take place in parallel.   The net of all this is that the 250GB 5400 RPM drive is actually quite a bit faster overall than the 100GB 7200 RPM drive, despite having a slower rotational speed.

So – faster (rotation) isn’t necessarily faster (overall).  And yes I am enjoying having an additional 150GB too :)

P.S. You might wonder why the newer bigger drives aren't also 7200 RPM.   I think it is because 1) they draw more power, and 2) they are noisier (whinier).   Both are important factors to laptop users.   So as long as the hard drive vendors can give you faster performance at a lower rotational speed, they’ll do it.

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