== What some _fdisk_ options actually do

Since I was just looking into this recently, I will write down what I've
learned about the less obvious effects of some of the _fdisk_ commands.

- The _u_ command: As a side effect of switching to using sectors as
  the size unit, _fdisk_ stops rounding up the end of your partitions
  to the nearest track or cylinder boundary.

- The _c_ command: This stops _fdisk_ from rounding up the start of
  partitions to the nearest track or cylinder boundary. (Fdisk gives
  this the cryptic description of turning off "dos compatibility".)

An example may help show the effects:
* with the default settings, a disk with just an extended partition
  that takes up the whole disk will have it start at sector 63 and
  end somewhat before the end of the disk.
* with _u_ alone, the partition will start at sector 63 but run to
  the end of the disk.
* with _u_ plus _c_, the partition will start at sector 1 and run
  to the end of the disk.

If you care about the exact size of your partitions, for example
because you are trying to get some number of exactly identically sized
partitions, you should use _u_. Using _c_ is optional but will get you
somewhat more disk space.

(Note that even with sector units and no DOS compatibility _fdisk_
will still silently add one sector to your logical partition sizes,
apparently so that partitions always start on an even sector
number. This is not visible in _fdisk_ output (which always reports the
size of partitions in Kbytes, regardless of the setting of _u_); you
have to use a tool like _sfdisk_ to see the exact details.)

I see no reason not to use _u_ and _c_ all the time, with the possible
exception of leaving some disk space at the front of disks for the
bootloader. The tracks and cylinders that _fdisk_ talks about are purely
imaginary constructs on any disk made since at least the turn of the
century, all of which have much more complicated internal geometries
and actively hide them from you anyways, so you might as well just use
straight sectors (or kilobytes, megabytes, and so on) and dispense with
all the complexity and artificial limitations.