Comparison of 7500k and 10k RPM hard drive performance


I just bought a new Dell XPS with 2x700GB 7500 RPM drives (ST3750528AS) in a RAID 0 array, and I moved over my (very expensive) 2x300GB 15,000 RPM RAID 0 array. This seemed like a good time to compare the performance of 7500RPM  and 15kRPM drives.

Now, there is one big factor to consider besides the drive spin speed: The newer 700GB drives are more than twice as big as my older 300GB drives, which means their data is about twice as dense. Therefore, they should be able to read data about twice as quickly, if the RPMs were equal.

First, the older, 15k drives:

The average read speed of 174 MB/sec is very respectable, but not astounding given the cost. It’s a RAID 0 array, which means my computer is reading from two disks simultaneously and in parallel, so it’s probably about twice the rate of an individual drive. My stand-alone and much less expensive ST31500341AS 1.5TB 7500RPM drive averages about 100 MB/sec.

Next, here’s the performance of two 7500RPM drives in a RAID 0 array:

They’re about 15% faster than the 15k drives, averaging almost 200 MB/sec. The slower rotational speed decreases their performance, but the higher density increases it.

They lose in Access Time, however. If the Transfer Rate (in MB/sec) is how long it takes you to read a book, the Access Time is how long it takes you to find the book on the shelf. If you were racing someone with a faster Transfer Rate and a slower Access Time, you would win any time you had to read a small book, and lose any time you had to read a big book. Therefore, the bigger, lower RPM drives are better at reading large blocks of data, while the smaller, higher RPM drives are better at reading small, random blocks of data.

As you can see from the HD Tune screenshots, the 15k array has an Access Time more than twice as fast as the 7500RPM array. This makes sense, because, access time is determined by how long it takes the drive to spin around to the requested section of the disk (disks always spin in one direction, and don’t stop while they’re in use). On average, it’s going to have to spin about half-way around to find the next random piece of data. That should only take 2ms on the 15k RPM drives, or 4ms on the 7500RPM drives. However, the drive head also has to move in toward the center of the disk or out towards the outer edge of the edge, and then wait for the disk to spin to the right spot, which can take longer.

Individual disks not in a RAID 0 array would have a Transfer Rate of about half the speed. However, individual disks not in a RAID 0 array would have about the same Access Time.

So, I made the 15k RPM array my system drive, because system files tend to be a lot of random access. I’m also using it for my temporary/scratch files, virtual machines, and Lightroom Catalog, because they are limited mostly by access time. I use the 7500 RPM array to store my photos, because they’re large, contiguous files that will benefit from a higher Transfer Rate more than a higher Access Time.

The Burst Rate is just the maximum speed of the interface. Basically, that’s how fast the drive could transfer something to the computer if it were already cached on the drive itself. Clearly the newer RAID array has better caching built-in, but that probably won’t make a huge impact on my performance, because Windows will be doing higher-level caching–I have plenty of RAM.

Here is more information about understanding hard drive performance.

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