[CentOS] Solved Re: imaging a drive with dd

Fri Mar 3 17:49:19 UTC 2017
Lamar Owen <lowen at pari.edu>

On 03/02/2017 11:57 PM, Robert Moskowitz wrote:
> The following worked:
> # dd if=/dev/sdb of=cubietruck.img bs=512 count=6268927
> 6268927+0 records in
> 6268927+0 records out
> 3209690624 bytes (3.2 GB, 3.0 GiB) copied, 114.435 s, 28.0 MB/s
> So bs= IS the drive blocksize.
> This is the result of trying a number of different values for bs and 
> count.

You can set bs to a multiple of 512 and it will go a lot faster.  If I 
have to use raw dd for cloning, I will factor the count all the way down 
to primes, and multiply the blocksize by all of the factors up to the 
largest prime factor. This is trivially easy on a CentOS system (factor 
is part of coreutils):

[lowen at FREE-IP-92 ~]$ factor 6268927
6268927: 7 43 59 353

So you could use 512 times any of these factors, or several of these 
factors.  I would probably use the line:
dd if=/dev/sdb of=cubietruck.img bs=9092608 count=353
Note that while dd can use the abbreviation 'k' you would not want to 
use that here since 2 is not one of the factors of your count.  A 
roughly 9MB blocksize is going to be loads faster than 512, but still 

Or you could make it easy on yourself and use either dd_rescue or 
ddrescue.  When I was working on the ODROID C2 stuff last year I built 
ddrescue from source RPM early on, before it got built as part of the 
EPEL aarch64 stuff.  Either of these two will figure out the optimum 
blocksize for you for best performance, and you get progress indications 
without having to have another terminal open to issue the fun 'kill 
-USR1 $pid-of-dd' command to get that out of dd.  The ddrescue utility 
for one includes a '--size=<bytes>' parameter so that you can clone only 
the portion you want.