Re: dd if=/dev/hda

[Jeff Licquia <jeff@licquia.org>]

On Mon, 2002-04-01 at 18:35, Charles Menzes wrote:
> when i was working at my last job, i saw a guy doing something of a 
> nightly "backup" on his system by doing
> 
>   dd if=/dev/hda of=/dev/hdc		
> 
> or something along those lines. i am not certain if there were any other 
> flags being used or not. regardless, i remember someone in the unix group 
> freaking out that it was being done. 
> what are the risks of such an action? the guy doing it claimed he was well 
> safe since the two disks being "mirrored" were identical, same brand, same 
> model, etc... the guy complaining, complained with facts that were frankly 
> way past my understanding. can someone comment on the why's not to do 
> this? or why its okay.

If the disks are identical models, and both drives have no partitions
mounted (or mounted read-only), then it's a bit dodgy, but should be
OK.  It also works well if the source drive is a CD, and you're trying
to copy data CDs.  (In this case, set the output to a file on a
filesystem with a lot of space.)

In any other situation, the possibility exists that the backup, as
copied, will be corrupt, as file activity on the drive may affect parts
of the disk already copied as well as parts yet to be copied.

It's generally a bad idea to do this for backup.  You can get the same
effect with mirroring (either using kernel RAID or a hardware RAID
controller), with the added bonus that the mirroring is real-time.  Tar
or cpio/afio are good alternatives, with amanda as an excellent
"enterprise-style" multi-system backup scheme.


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