Do HDD's get slower as you fill them up? Why?

Rafael Mestdag · Feb 20, 2017

I'm asking because I've just short-stroked my WD 500GB down to 150GB and it is very fast to start with but it seems to be getting slower and slower as I fill it up. In that case, how much of the drive should I fill up until it starts to slow down?

Is this true of any HDD, short-stroked or not? Why?

SBMfromLA · Feb 20, 2017

HDD need to a good Defrag at least weekly to keep them running smoothly. Also, try to keep at least 15-20% of the drive free.

Solandri · Feb 20, 2017

1. Even if you don't short-stroke the drive, filesystems are designed knowing that HDDs are faster on the outer tracks. So they try to fill those tracks first (especially important since the OS is one of the first things written to the drive). As the drive fills up, most of the new files are written to the inner tracks, and thus reading and writing them is slower.

2. As the drive fills up, the empty space becomes increasingly broken up and scattered (files are written with a little empty space at the end in case you edit the file and make it longer in the future). As it becomes harder to find enough contiguous space to write a file, the newer files become more and more fragmented, which drastically slows down their read/write speeds. Modern OSes combat this by running defragmenters during idle time. But as the free space drops below 25%, defragmenters become less and less effective (they have to move files multiple times to make enough free space to write them as continuous blocks).

Rafael Mestdag · Feb 20, 2017

Solandri :

1. Even if you don't short-stroke the drive, filesystems are designed knowing that HDDs are faster on the outer tracks. So they try to fill those tracks first (especially important since the OS is one of the first things written to the drive). As the drive fills up, most of the new files are written to the inner tracks, and thus reading and writing them is slower.

2. As the drive fills up, the empty space becomes increasingly broken up and scattered (files are written with a little empty space at the end in case you edit the file and make it longer in the future). As it becomes harder to find enough contiguous space to write a file, the newer files become more and more fragmented, which drastically slows down their read/write speeds. Modern OSes combat this by running defragmenters during idle time. But as the free space drops below 25%, defragmenters become less and less effective (they have to move files multiple times to make enough free space to write them as continuous blocks).

On that basis, you'd say an HDD will start to slow down as it reaches about 75% of filled space?

Solandri · Feb 20, 2017

Rafael Mestdag :

It will start to slow down the moment you start to write data to it. But the rate it slows down begins to increase dramatically once you pass about 75%. With aggressive defragmenting, you can probably push that up to 80%-85%.

It partly depends on what sort of files you have on it though. If the only thing you have on the drive are large movie files, then you could probably get up to 90% before running into fragmentation slowdown. OTOH if you've been torrenting files and didn't enable the "pre-allocate" option, the torrent program will write each fragment it downloads to a different spot on the drive. And you can badly fragment your drive even with as little as 10% of the space filled.

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Do HDD's get slower as you fill them up? Why?

Rafael Mestdag

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Rafael Mestdag

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