Why is my hard drive slowing down halfway through a read/write benchmark?

Question

I'm using gnome-disks to do a benchmark on an old 5400RPM Hitachi HDD that I pulled from a PS3. It's connected to my computer through an external USB 3.0 enclosure. Here's the output graph from the benchmark:

Why does the speed start steadily decreasing about halfway through the benchmark? I ran all the SMART self-tests and it said everything was fine, the drive is healthy. So what's wrong?

On another note, 35MB/s seems a bit slow. Don't even 5400 RPM hard drives generally run faster than that? Is it because I'm using an external USB enclosure and not connecting directly to SATA?

Answer 1

That's because harddisks use "zone density recording". In plain words: there is more data on one rotation on the outside of the disk than on the inside. With a constant speed, a disk gives you more data on the outside (left side of your graph) than on the inside.

See this excellent link. I quote:

An interesting added benefit from zoned bit recording is that the raw data transfer rate of the disk, also referred to as the media transfer rate (a bit of a misnomer), when reading the outside cylinders is considerably higher than when reading the inside ones. Although the angular velocity of the platters is constant regardless of which track is being read, the outer cylinders contain more data. Bear in mind though that angular velocity does not necessarily compensate for the fact that the outer tracks (periphery of the platter) is moving much faster than the tracks at the core of the platter.

Take note that constant angular velocity is not the case for all drive technologies, such as older CD-ROM drives.

Since data is written to the outer tracks of a drive first, hence the drive is filled with data from the outside in. The fastest data transfer occurs when the drive is first used and data retained in the outer tracks. Many people that perform benchmarks on their systems and their hard drives when new, then make some tweaks and changes to their system only to return to their benchmarks weeks or months later only to be unpleasantly surprised that the disk and its benchmarks are getting slower. Actually, the disk has probably has not changed at all, but the second benchmark may have been run on tracks closer to the center of the disk. While most people that take benchmarking seriously defragment their drives before running the tests, fragmentation of the file system can have impact performance benchmarks.

Funny sidenote: harddisks and vinyl records are played from the outside to the inside, but CD's are played from the inside to the outside. Funny world, huh?