[barrkel]

You have not used a file system exceeding 90% use with large files and lots of little files coming and going.

ZFS in particular completely falls off a cliff somewhere between 80% and 90%, due to the copy on write nature of ZFS always allocating and freeing small bits of space. That creates the little gaps all over the FS which murder performance when the big gaps run out.

[lathiat]

The old default for this was lower but most modern versions of the ZFS defaults don’t hit this until the vicinity of 96%

It’s not technically the little holes or even CoW that is the problem. It simply switched from a “first fit” to a “best fit” algorithm as it got full which was quite expensive to do the search for.