hope you're not running -o compress=lz4 , because you are going to be in for a big surprise when you try to pull this emergency lever! you may be shocked to see you don't actually get much space back!

i do wonder how many FS would actually allocate the 8GB if you, for example, opened a file, seeked to 8GB mark, and wrote a character. many file systems support "sparse files"[1]. for example on btrfs, i can run 'dd if=/dev/zero of=example.sparse count=1 seek=2000000' to make a "1GB" file that has just one byte in it. btrfs will only allocate a very small amount in this case, some meta-data to record an "extent", and a page of data.

i was expecting this article to be about a rude-and-crude overprovisioning method[2], but couldn't guess how it was going to work. SSDs notably perform much much better when they have some empty space to make shuffling data around easier. leaving a couple GB for the drive to do whatever can be a colossal performance improvement, versus a full drive, where every operation has to scrounge around to find some free space. i wasn't sure how the author was going to make an empty file that could have this effect. but that's not what was going on here.

[1] https://wiki.archlinux.org/index.php/sparse_file

[2] https://superuser.com/questions/944913/over-provisioning-an-...