bug#10755: stat: Please add directive to display size on disk

[Filipus Klutiero]

The other day I tried figuring out how much disk space a small file 
took. I used stat, but I only realized today that stat does not provide 
that information directly, as explained by Eric Blake and Pádraig Brady 
in http://debbugs.gnu.org/cgi/bugreport.cgi?bug=10561

This information is provided indirectly by %b and %B. Multiplying the 2 
should give the size on disk. Example:

stat -c 'allocated-space=%B*%b apparent-size=%s' $file

Currently, this information can be obtained from du:

$ stat -c '%s' /etc/phpmyadmin/htpasswd.setup
54
address@hidden:~$ du -h /etc/phpmyadmin/htpasswd.setup
4,0K    /etc/phpmyadmin/htpasswd.setup
address@hidden:~$

However, it would be quite useful if stat could be used to display 
simultaneously a file's real size and size on disk.


Eric Blake suggested this directive to be %S. For the description, he 
suggested:

%S  Allocated size (same as %b * %B)

I would like to aim for clarity, as these various statistics can be 
confusing. I suggest to use the name "Size on disk". I also suggested 
the following:

Size occupied when including slack space
Size of the clusters occupied

I suggest the following synthesis:

Allocated size (space allocated on the storage device, same as %b * %B). 
This is the size of the allocated clusters, which includes slack space.