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FreeBSD, Linux, Technical

Why Hard Drives Get Slower As You Fill Them Up

I built a new system (from old parts) yesterday. It is a RAID10 with 4x 8TB disks. While the system is up and running, it takes forever for mdadm to complete “building” the RAID. All the same, I got a little smile from this graph:

As the RAID assembles, the R/W operations take longer to complete.

The process should be about 20 hours total. As we get move through the job, the read-write operations take longer. Why? I have noticed this before. What happens here is that the disk rotates at a constant speed, but towards the rim you can fit a lot more data than towards the center. The speed with which you access data depends on whether you are talking to the core or to the rim of the magnetic platter.

What the computer does is it uses the fastest part of the disk first, and as the disk gets filled up, it uses the slower parts. With an operation like this that covers the entire disk, you start fast and get slower.

This is part of the reason that your hard drive slows down as it gets filled up: when your disk was new, the computer was using the fast-spinning outer rim. As that fills up, it has to read and write closer to the core. That takes longer. Another factor is that the data might be more segmented, packed into nooks and crannies where space is free, and your Operating System needs to seek those pieces out and assemble them. On a PC, the bigger culprit is probably that you have accumulated a bunch of extra running software that demands system resources.

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