dd question

Polytropon freebsd at edvax.de
Sun Sep 27 15:40:23 UTC 2015

On Sun, 27 Sep 2015 10:09:04 -0453.75, William A. Mahaffey III wrote:
> On 09/27/15 09:49, Polytropon wrote:
> > Take the disklabel editor screen as an example. Make sure
> > 'c' is as big as the whole device, then take the end of
> > the last partition, 'a', as an offset, and add the desired
> > size for 'd'.
> Hmmmm .... My 'c' slice is *not* the size of the whole drive:
> [root at kabini1, /etc, 9:54:38am] 596 % bsdlabel da0
> # /dev/da0:
> 8 partitions:
> #          size     offset    fstype   [fsize bsize bps/cpg]
>    a:    1401120          0    4.2BSD        0     0     0
>    c:    1401120          0    unused        0     0     # "raw" part, 
> don't edit
> [root at kabini1, /etc, 9:54:44am] 597 %

Yes, and that's why you're seeing a partition occupied
by data at 100%, but smaller than the USB drive itself.
However, 'c' _can_ be the size of the media. It's not
a problem when 'a' is the same size as 'c', which only
means that the first partition is bootable and spans the
whole available space.

> 1401120 (sectors) * 512 (bytes/sector) = 7.173734e+08 Bytes or 717-ish 
> MB (700.56 MiB), which is what I'm seeing out of my mounted drive, not 
> the 4-ish GB (3.61-ish GiB) it should be ....

Yes, that's the approx. 650...700 MB image which contains
the partitioning information at the beginning. Keep in mind
that this image, applied with dd, doesn't just contain data;
it contains "meta-information" such as a boot sector and a
partition table. This information is for the 700 MB amount
the image requires, so the 4 GB media "looks smaller".

> I'm guessing the drive 
> would boot & install OK, however it is convenient to be able to log the 
> proceedings in case of questions, that is what I am getting at ....

That's a very useful feature - being able to mount an additional
partition on the USB stick (no network required!) and use
"script" or another program to store error logs or outputs
(of "dmesg", "usbconfig" or other tools).

Of course mounting the partition for access is a task up to
the user, the installer will ignore it, but once it is there,
it can be used, for example, if you want to copy some files
before you start the installation, of have files ready you
want to transfer after installation...

Magdeburg, Germany
Happy FreeBSD user since 4.0
Andra moi ennepe, Mousa, ...

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