bug#7042: df --help does not show `-m' option

[Paul Eggert]

On 09/16/10 11:21, Eric Blake wrote:
> document -m, with --block-size=M as the long-option spelling
> add -g, with --block-size=G as the long-option spelling

This doesn't sound like a good idea.  Multi-terabyte disks
are already here, and "df -t" is already taken.  Also,
"df -g" already means something different in Solaris.

Is it really that hard to type "df -Bg"?

If we're going to make incompatible changes, I suggest that
we solve the problem once and for all, by having "df" choose
the default blocksize dynamically, based on the size of the
output line describing the smallest disk.  For example, where
"df" currently outputs this:

Filesystem   1K-blocks      Used Available Use% Mounted on
/dev/sda1    11620338002   1437021 11618900981   1% /r/opt
/dev/sda2     20971520   1335871  19635650   7% /home/eggert

"df" would notice that the smallest file system is between 1GB and 1TB,
so it would default to 1 GB blocks, as follows:

Filesystem  1GB-blocks      Used Available Use% Mounted on
/dev/sda1      11900GB       2GB   11898GB   1% /r/opt
/dev/sda2         22GB       2GB      21GB   7% /home/eggert

This is much more useful as an output format, because one can visually
see which file systems are larger by seeing how many digits are there.
Contrast this to the output of df --si:

Filesystem     Size   Used  Avail Use% Mounted on
/dev/sda1       12T   1.5G    12T   1% /r/opt
/dev/sda2       22G   1.4G    21G   7% /home/eggert

which is harder to visually parse that way.

While we're at it, we should fix "df" so that its columns line up better;
that would be another incompatible change, but it'd be worth it
on hosts where the file system names are long.