Why should a maximum of 80% of hard disk space be used?

[10:55 Tue,2.January 2018 by Thomas Richter]

A general rule is to never use more than 80% of the storage capacity of a hard disk - but hardly anyone is keeping to it, why shouldn&t you use valuable hard disk space, for example 200GB of 1000 GB or 1 TB of 5 TB of storage space?

This article explains the background of the 80% rule on the basis of basic technical knowledge. It is important because the performance of hard disks decreases dramatically with decreasing storage space. Although this performance decrease differs from model to model, it is clear for every HDD. The reading speed of a 2 TB Western Digital (WD2003FYYS) HDD is 101.62 MB/s on the outside and then decreases to 78.70 MB/s when using 50% of the space on the inner tracks and to 49.02 MB/s at 90% - slightly more than halving the performance in the inner sectors.

The reason for this is a physical one: due to the ring-shaped arrangement of the data in tracks, more information fits on a track the further it is positioned at the edge of the disc. With a single revolution, more data is read within the same time period (i. e. the speed of data transfer is higher) because the track read is longer.

Data Structure of a Hard Disk Drive

Since hard disks are therefore always written from the outside to the inside, the speed decreases the further inside the data is located, i. e. the more full it is. The 20% rule gives a rough indication of the amount of filling, above which the read and write speed of a hard disk declines considerably. Another advantage of free storage space - enough space for defragmentation of data.

In the end, the user only has to weigh up the speed and storage space - either the available storage space is used to the maximum at the expense of the speed - or the goal is a continuously high speed, for which storage space must be sacrificed - in other words, the result must always be 20% more memory.

Although 80% applies only to HDDs, not SSDs - but SSD users should follow a similar rule (for other technical reasons). Most SSDs already organize the storage space in such a way that about 10-15% remain hidden - depending on the model (due to the fact that

overprovisioning provides for completely different size of invisible reserves). Sometimes e. g. with Samsung SSds you can determine the size of the overprovisioning yourself. However, the reason for this is not the fragmentation or sectoring, but a reserve for failures, between 0-28% memory cells and for trimming - the reduction in speed is not as great as with HDDs. However, it is still advisable to have a storage space reserve of at least 10%.

Translated with www.DeepL.com/Translator

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deutsche Version dieser Seite: Warum sollte man maximal 80% des Speichers von Festplatten nutzen?