On FreeBSD systems, and presumably a bunch of others, there is an amount of space reserved for root which is MINFREE% of the total capacity. With multi-terabyte filesystems, the default of 8% is a staggering amount of space. Volumes that large could have less than 1% MINFREE, but of course, it must be an integer value.
Will this ever change? Does anyone even make use of MINFREE anymore?
The key point missing from the other current answers is that the minfree space is not solely reserved for root. While it is true that the root user is exempt from minfree restrictions, this space is not there just for this reason.
The space reserved for minfree is used by the filesystem code to keep performance good - in particular, it keeps fragmentation on the filesystem down. The value of minfree also affects whether the filesystem tries to be fast or space-efficient by default; if minfree is below 5%, and you haven't altered the optimisation method using the -o flag to tunefs(8), you'll see filesystem performance drop.
This comes up from time to time on the FreeBSD lists; there is a reasonable summary (and thread) from May 2003. There is a last word not attached to that page, too:
"The steady state of disks is full"
-- Marshall Kirk McKusick
As you say, in these days of huge filesystems, a small percentage can turn out to be a huge amount.
I guess it depends on your setup. On my users' /home partition I set it to 0. Don't need any reserved space for root there. On system partitions (which generally tend to be smaller anyway) I'll still leave some reserved space, in case of... you know, gremlins 'n stuff.
One percent is still one percent, no matter how big the total is. If you have such a staggering amount of space as you say, why so hung up over that last one percent? I wouldn't worry about it too much, if your disk is 99% full you need some new disks anyway, that extra 1% isn't going to help you much.
