Re: Write to DVD-RAM

On 7/25/19 7:14 PM, Zhi-Qiang Lei wrote:
> On Jul 25, 2019, at 10:24 PM, gwes <gwes@oat.com> wrote:
>>
>> On 7/24/19 10:19 PM, Zhi-Qiang Lei wrote:
>>> Hi, I'm trying to encrypt a DVD-RAM before putting some files onto it on my OpenBSD 6.5 desktop. But neither dd nor disklabel seems able to work on the drive. Did I miss something?
>>>
>>> $ dmesg | grep cd
>>> cd0 at scsibus3 targ 1 lun 0: <HL-DT-ST, BD-RE BU40N, FR07> ATAPI 5/cdrom removable serial.13fd3940302020202020
>>> cd0 at scsibus3 targ 1 lun 0: <HL-DT-ST, BD-RE BU40N, FR07> ATAPI 5/cdrom removable serial.13fd3940302020202020
>>>
>>> $ doas dd if=/dev/urandom of=/dev/rcd0c bs=1k
>>> dd: /dev/rcd0c: Invalid argument
>>> 1+0 records in
>>> 0+0 records out
>>> 0 bytes transferred in 0.000 secs (0 bytes/sec)
>>>
>>> $ doas disklabel -E cd0
>>> cd0> a
>>> partition: [a]
>>> offset: [0]
>>> size: [2236704]
>>> FS type: [4.2BSD]
>>> cd0> w
>>> cd0> p
>>> OpenBSD area: 0-2236704; size: 2236704; free: 0
>>> # size offset fstype [fsize bsize cpg]
>>> a: 2236704 0 4.2BSD 2048 16384 1
>>> c: 2236704 0 unused
>>> cd0> q
>>> No label changes.
>>>
>>> The same drive can be formatted and used on Mac OS X.
>>>
>>> Thanks and best regards,
>>> Siegfried
>>>
>> Did you try 2K blocks? The low level of CDROM only works that way.
>>
>
> Blocks larger than or equal to 2k get a "dd: /dev/rcd0c: short write on character device". Regarding to cd(4) I thought the device is readonly, so dd(1) and disklabel(8) cannot write on it, but fdisk(8) works fine.
>
> $ doas dd if=/dev/urandom of=/dev/rcd0c bs=2k
> dd: /dev/rcd0c: short write on character device
> dd: /dev/rcd0c: Invalid argument
> 1+0 records in
> 0+1 records out
> 512 bytes transferred in 0.008 secs (57960 bytes/sec)
>
> $ doas dd if=/dev/urandom of=/dev/rcd0c bs=512
> dd: /dev/rcd0c: Invalid argument
> 1+0 records in
> 0+0 records out
> 0 bytes transferred in 0.000 secs (0 bytes/sec)
>
/dev/cd0 is likely a symbolic link to something else in /dev.
It's not clear what's going on unless we know exactly what's being used.
"cd0" is not a usual OpenBSD device access even though one sees
that in dmesg.

OpenBSD disk-like devices are usually referenced in the very
old style which distinguishes "raw" [unbuffered direct to device]
from "cooked" [system buffered]. This differs from at least Linux practice.
Dunno about other BSDs or Macs.
Buffered devices are essentially only used to mount as filesystems.

A raw device is /dev/r<controller-or-generic-type><unit-number><partition>
A buffered device is
/dev/<controller-or-generic-type><unit-number><partition>
Note that there is always a partition letter.
The kernel will always emulate a 'c' partition = whole device if necessary.

So the most specific way to refer to your cd device is /dev/rcd0c.

As a convenience and to reduce operator errors, many system maintenance
programs will deduce /dev/r<specified-device>c from a bare device
like sd0. This can be confusing to people new to OpenBSD.