However, for an SSD, you should keep between 20%-25% free space to get the best performance and write life, especially the latter. This benchmarks measures performance (on the y axis) vs used space (on the x axis):įor best operation, you should keep 10-15% free space (empty space) on an HDD. Here you can see an example of this effect. For this reason, writing new data will be slow, unless space near the outside is reopened by deleting files, and reading will be slow from the inside (new data), but would remain fast for 'old data' that's stored near the outside. Due to HDD disks having uniform data density, this means that as they spin, fewer bits are moving under the read/write head per second vs what happens when reading from or writing to the outside edge of the disk.

Therefore, by the time the drive is nearly full, the new data you're adding is being put on the inside of the disk. By default, HDDs will write to the outside of the disk (platters), moving inward as more data is stored. HDDs will lose read and write speeds as they fill up, but for a different reason. I would like to provide an example of this effect the way I have for the HDD below but sadly it seems most people only test the first ~10% of their SSDs or so, and as a result any images you see of this test will show no degradation in performance.

This shouldn't really be noticeable until you get it ~70% full or more. Read speeds however shouldn't be noticeably impacted. This of course is slower, and will wear the drive out faster. This is because they aim to write new data to unused space whenever possible, but as they fill and this becomes impossible, they have to go back to space that is already used and add more to it, which involves reading from it, adding the new data, then writing that back to the SSD. SSDs will wear faster and lose write speed as they fill up.

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