Backup with dd?

Malcolm Kay malcolm.kay at
Mon Jan 3 18:26:55 PST 2005

On Tue, 4 Jan 2005 04:27 am, Eric F Crist wrote:
> Hello all,
> I've decided to try doing a complete system backup, attempting a
> bit-for-bit copy.  A friend told me to try the following:
> # dd if=/dev/ad4 of=/dev/ad6
> Both drives are identical SATA150.  Is this the best way?  I'm hope to
> be able to do a daily/weekly backup this way, and if my primary drive
> fails, switch the cables and just reboot.

There can be a few major problems in using dd for backup:

If the source is mounted then it can change during backup and lead to
a bckup that is not self consistent. At very least when you attempt 
to boot the backup you will find it is not marked clean and fsck will
spend some time fixing it. This problem can probably be avoided by 
having the source (and destination) unmounted during the backup;
perhaps by running the backup from a "fixit" system.

The other problem is that it normally will take a very long time.
I've tried this in the past -- can't remember details of disk sizes
etc. but do recall times in excess of 20 hours, during which the system
should be otherwise as dormant as possible.

Identical drives are like identical twins; ie they are not quite 
identical. While it is not likely to be a problem it could be.
Safer to use equal sized slices on each drive rather than complete drive.

My preference would be to use dump and restore on a partition by 
partition basis; which is more or less the intended application of 
these utilities.


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