Bug#678015: debian-installer: Guided partitioning took 26 hours to complete erasure of encripted LVM volume

[rjs@netti.fi (Riku Saikkonen)]

I ran into a variation of this bug while test-installing wheezy: I was
installing on an external USB hard drive (because I didn't want to touch
the installation on the internal drive when only testing), and the
"erasing data" step took a very long time. I wasn't in a hurry and left
it running overnight so I'm not sure how long, but more than 6 hours
(for a 320 GB drive). I did expect it to take a while, since USB 2.0
limits write speed anyway.

I suggest including a few lines of explanation to the erasing screen (it
has a lot of room for extra text, at least in the text-mode installer).
I think it would be good to explain these three points:

 - Does this step write random data or only 0s to the disk? (I think
   messages #15 and #35 in this bug report show that this is not clear
   to the users.)

   Hmm, many messages in this bug report imply that it writes 0s.
   However, I tried using dd on the system I installed and that seems to
   point to random data: a command like

     dd if=/dev/sdb5 skip=120000000 bs=1K count=10 | hd

   shows random-looking data for a variety of "skip" offsets (e.g., the
   above command should read at an offset of 120*10^6 * 1024 = 120GB),
   and df shows that I only use 14GB of 295GB, so I think at least some
   of the offsets I tried should be on an ununsed area of the file
   system.

   So I'm confused now. Maybe someone reading this knows what happens?

 - Can the "Cancel" button be used to skip (the remainder of) this step,
   or will it just go back to the menus such that proceeding will
   restart this step from the beginning?

   (Most of the "Cancel" buttons in the installer don't skip steps but
   only give ways of restarting them after e.g. changing options. But
   messages #25 and #30 suggest that this one is an exception...)

   Hmm, maybe you could just rename "Cancel" to "Skip the remainder of
   this step and continue" (or include both buttons if it seems useful)?

 - And possibly include a warning that it is normal for this to take
   many hours on some hardware, and briefly explain why it is done.