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

From: Pádraig Brady
Subject: bug#7042: df --help does not show `-m' option
Date: Fri, 17 Sep 2010 09:56:57 +0100
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv: Gecko/20100227 Thunderbird/3.0.3

On 16/09/10 23:34, Paul Eggert wrote:
> 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"?

Yes I agree the current unit selection options are fine.

> 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.

That would break lots of scripts I'd say
(they should use -P, but many don't).
In any case I don't think there is enough benefit
in such a format change given the common wide range
of device sizes attached to systems.
Personally I find the `df -h` format easiest to read.

> 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.

We definitely should do this to support languages
with side column headings, and `df -B\'1`
I've an unfinished local branch to do this with mbsalign()


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