Disks: why size means performance

Big hard drives have performance benefits that are at least as important than their raw capacity - and they may be the best and cheapest upgrade you can make to your system. Here's why.

Big=fast Most people keep less than 80 GB of data, including the operating system and their applications, on their hard drive. So why should they buy a 500 GB, 1 or even 2 TB hard drive?

One simple reason: speed. That big hard drive will give you this snappiest performance this side of a solid-state disk.

For many applications, even faster than a costly SSD. For a lot less money.


Big circles, little circles Data is laid on disks in blocks called sectors. The sectors are laid down in circular tracks.

Disk engineers saw a long time ago that they could put more sectors on the outside of a disk than on the inside. As the head gets closer to the center of the disk, there are fewer sectors - and your data rate slows down.

How much? The innermost track will commonly have only ~45% of the speed of the outermost track.

But that's not all Bigger hard drives have another advantage: higher bit density. So for the same RPM, more bits come out.

The difference is substantial. A 3.5" 500 GB drive might max out at 80 MB/sec, while a 1 TB drive will reach 100 MB/sec. The new 2 TB drives can reach 140 MB/sec.

1 more thing Not only do big drives deliver more bandwidth, they also deliver more I/Os per second (IOPS). On a half full drive the head will never have to move across the entire platter to access data, cutting seek times - all else being equal - in half.

Big million dollar enterprise RAID arrays stuffed with 15k drives frequently short stroke their already-fast drives to make them even faster. It works on home systems too.

What about SSDs? Flash-based solid state disks (SSD) excel at random reads - which is why they boot up a system so much faster than a hard drive. But due to the housekeeping needed to make an SSD look like a hard drive their write performance isn't nearly as stellar.

A state-of-the-consumer-art SSD, the 80 GB Intel X25-M G2, is spec'd (pdf) at 70 MB/sec for sustained sequential writes.

A 500 GB Seagate Momentus 7200 rpm notebook drive will average almost 80 MB/sec while offering 6x the capacity for a much lower cost.

If you work with large files - photography, music, video - you'll notice the difference over an SSD.

The Storage Bits take Installing a big hard drive is, after more RAM, the easiest performance upgrade most of us can make. Even if you don't need the capacity you'll appreciate a friskier computer.

Another benefit: after initial break-in, new hard drives tend to be much more reliable than a 3 year old drive. Faster, more reliable and cheaper than an SSD, a hard drive upgrade may be all you need to keep your system happy.

Courteous comments welcome, of course. For more on this topic see Hard disks do get slower with use.