Re: disk geometry issues when trying to set up encrypted partition

On Wed, 16 Jun 2010, Harry Palmer wrote:

> Hi there.
>
> I'm fairly new to openbsd and I'm hoping someone with better
> understanding than me of how its disk handling works can help.

hi Harry,

i'm far from an expert but will share what i know.

> Beginning my effort to encrypt a 300GB drive in a 64bit Ultrasparc,
> I followed these initial steps:

i'm also into using 64bit Ultrasparc architecture.

> 1. used disklabel to create a single slice "a" on the drive
>
> 2. made a file system with newfs (is it necessary to have so many
>   backup superblocks?)

think they are hard set (and necessary for certain recovery senarios).

> 3. mounted sd2a on "/home/cy" and touched it with an empty file
>     "/home/cy/cryptfile"
>
> 4. zeroed out the file (and efectively the drive) with
>     "dd if=/dev/zero of=/home/cy/cryptfile bs=512"
>
>
> Here's the (eventual!) output of (4):
>
> /home/cy: write failed, file system is full
> dd: /home/cy/cryptfile: No space left on device
> 576520353+0 records in
> 576520352+0 records out
> 295178420224 bytes transferred in 19810.722 secs (14899932 bytes/sec)

that looks fine.   filled the whole "allocated" space in the a partition.

> Now I have:
>
> # disklabel sd2a
> # /dev/rsd2a:
> type: SCSI
> disk: SCSI disk
> label: MAW3300NC
> flags: vendor
> bytes/sector: 512
> sectors/track: 930
> tracks/cylinder: 8
> sectors/cylinder: 7440
> cylinders: 13217
> total sectors: 585937500
> rpm: 10025
> interleave: 1
> boundstart: 0
> boundend: 585937500
> drivedata: 0
>
> 16 partitions:
> #                size           offset  fstype [fsize bsize  cpg]
>   a:        585937200                0  4.2BSD   2048 16384    1
>   c:        585937500                0  unused

c is from unix tradition and represents the entire surface of the disk
in a single slice.

> and:
>
> # ls -l /home/cy
> total 576661216
> -rw-r--r--  1 root  wheel  295178420224 Jun 16 03:39 cryptfile

same as dd reported.

> and:
>
> # df -h
> Filesystem     Size    Used   Avail Capacity  Mounted on
> /dev/sd0a     1007M   44.8M    912M     5%    /
> /dev/sd0k      247G    2.0K    235G     0%    /home
> /dev/sd0d      3.9G    6.0K    3.7G     0%    /tmp
> /dev/sd0f      2.0G    559M    1.3G    29%    /usr
> /dev/sd0g     1007M    162M    795M    17%    /usr/X11R6
> /dev/sd0h      5.9G    212K    5.6G     0%    /usr/local
> /dev/sd0j      2.0G    2.0K    1.9G     0%    /usr/obj
> /dev/sd0i      2.0G    2.0K    1.9G     0%    /usr/src
> /dev/sd0e      7.9G    7.7M    7.5G     0%    /var
> /dev/sd2a      275G    275G  -13.7G   105%    /home/cy
>
>
>
> I have no understanding of this. I've never seen a df output
> that tells me I'm using 13GB more space than the drive is
> capable of holding.

slight misundertanding.  df reports usage with an allowance for warning
before file system is exhausted, so the default capacity is 105% and
for proper operation, shouldn't be used beyond 100%.     i believe that
as a user you can't fill it more than 100% (as reported by df) but root
can fill it beyond (necessary for some recovery senarios).

> I ask here because there's obviously potential for me to lose
> data somewhere down the line. I'll be grateful if anyone can
> explain where I've gone wrong.

think you did everything right and will feel right at home with OpenBSD
after you take into account the reserved 5% of filesystem capacity built
in to df.

-ron