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

Quoting Otto Moerbeek <[email protected]>:

> On Wed, Jun 16, 2010 at 05:45:40PM -0400, George Morgan wrote:
>
>> Quoting Harry Palmer <[email protected]>:
>>
>> >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.
>> >
>> >Beginning my effort to encrypt a 300GB drive in a 64bit Ultrasparc,
>> >I followed these initial steps:
>> >
>> >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?)
>> >
>> >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)
>> >
>> >
>> >
>> >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
>> >
>> >
>> >and:
>> >
>> > # ls -l /home/cy
>> > total 576661216
>> > -rw-r--r--  1 root  wheel  295178420224 Jun 16 03:39 cryptfile
>> >
>> >
>> >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.
>> >
>> >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.
>>
>> I've seen the greater than 100% full on a UFS? filesystem before when
>> you exceed the size of the filesystem.  There is space in the
>> filesystem for "lost+found" and all those superblocks? you were
>> complaining about that can get overwritten if you write too much to a
>> partition.
>
> Spoace for superblocks and other metadata is subtracted from available
> blocks. lost+found is an ordinary directory.
>
>>
>> So setting up your "dd" to actually stop before you overfill the
>> filesystem is what you need to do. (using bs=# count=# ... info you
>> can get before you start initializing your file with the df command
>> without the "-k or -h" to get number of blocks and block size)
>>
>> I'm sure the fine people on these lists will correct me if I'm wrong
>> in my assumptions...  :-)
>
> You are wrong, there;s no such thing as overfilling a filesystem. It's
> just the 5% reserved for root. An ordinary user runs out earlier. It's
> in the FAQ.
>

Sorry for the misinformation.  Thanks for the education.

George Morgan