Questions about the dd command
dan at spore.ath.cx
Sun Mar 2 02:18:36 UTC 2008
On Fri, 29 Feb 2008 14:24:55 -0700
Karl Larsen <k5di at zianet.com> wrote:
> elmo wrote:
> > I've spent literally hours trying to find direct answers on the
> > dozens of websites that discuss 'dd'
> > but the couldn't find what I wanted.
> > A few questions about the ' dd ' command:
> > For example: sudo dd if=/dev/sda2 of=/dev/sdb2 bs=1440
> > 1. Which is preferrable; execute the command from an installed
> > ubuntu (/dev/sda2) or from a ubuntu CD?
make sure you have the same partition and unit size, elmo! for more
info, see my write-up on the subject :
http://spore.ath.cx/~dan/doc/xpmove.html Applicable to any system, of
> There is no point to using dd from a CD-Rom. That is silly.
No, it's not at all silly. Elmo, please look closely at the other
responses to see why. In short: data consistency!
> > 2. How do you modify the example to show the data as it is
> > being copied?
it's possible in many ways; for one, using tee could do it.
> > Would this have a drastic effect on how long it takes to
> > complete the copy
> > compared to just watching a blinking cursor?
YES!!! I would expect that it would slow things down significantly.
My initial testing shows 1.1MB/S transfer rate
for `dd if=/dev/hda1` (outputs to terminal) and almost 50MB/S if I send
it to /dev/null. Translation: even an X terminal can only go about 1M
characters a second.
> There is no way I know to see what is happening.
'what's happening' can be seen with USR1 signal, as explained above.
The probably easiest incantation is:
$ killall -USR1 dd
incidentally, HUP will terminate dd! Don't use HUP!
> > 3. What effect on the does the ' bs = ' number have on
> > the time to complete the copy or the accuracy of the copy?
> > Is there a
> > default number if the the bs= is omitted from the command?
> > If you do not use a bs= it uses the default which is dead
> > slow. I use bs=10000 and it is much quicker.
Elmo, I recommend you use the block size of your hard disks.
> Suggest you read man dd where all these things come from.
the man pages aren't always the cornucopia of information people tout
them as - not that I don't read them all the time. Many appear to be
moving towards gnu info.
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