dd question

[KOERSNER]

        Hello

        We normally use dd on Compaq Tru64 unix to make
        copies of disks including partition information and data
        from all partitions. It even works for the system disk.
        We have tried to do the same thing in RedHat Linux 7.2 but with
        problems. 1) It takes relatively long time, ca 15 h for 18 GB.
        2) The operation ends with an error message ( No such device
        or address). 3) If we try to copy the system disk in this way it will
        not be a working bootable system disk.
        Is this due to the fact that we are optimistically
        trying to copy a mounted 'living' (system)disk or is this some kind
        of problem with 'dd' ?

        Sincerely Ib Koersner
        The Svedberg Laboratory
        Uppsala
        Sweden


Copying whole disks using dd with raw devices:
First connect raw devices to block devices:
 raw /dev/raw/raw1 /dev/sda
 raw /dev/raw/raw2 /dev/sdb
Then copy:
[1]-  Running                 time dd if=/dev/raw/raw1 of=/dev/raw/raw2 bs=2048 
&
address@hidden root]# dd: reading `/dev/raw/raw1': No such device or address
8891619+1 records in
8891619+1 records out

real    891m9.305s
user    0m8.470s
sys     4m8.530s