Friday, June 4, 2010

When Raid mirroring just won't work - dd to the rescue

Having worked with many businesses on backup procedures, I try to adopt best practice on my own computers.

With this in mind, I have backup copies of entire hard drives which I refresh once or twice a year. This means that in the event of a critical failure, I can quickly grab the backup hard drive and put my main desktop pc back to the state it was in around 6 months ago.
     ( I back up business critical data using additional procedures than those discussed here )

Motherboards tend to have a raid controller, and raid mirroring procedure accessible in BIOS or startup screen.

With a similar sized disk I just select source and mirror and let the motherboard look after creating a backup (one time raid mirror). This always worked until recently* when I had to revert to using dd.

In defense of the 26 hours backup time, this is running dd via a system rescue usb stick on a 5 year old socket 754 motherboard, with SataI rather than SataII maximum throughput. Even so the 15.7 MB/s is shockingly slow, but for a one time job, then maybe that is not such a problem.

*Having bought two matching Samsung F2 1.5TB drives, I was surprised to find the Via onboard raid controller complaining that the drive sizes did not match. I can only assume that a 2005 raid controller, might not, in all cases, be good for properly recognising the huge terrabyte plus disks that we can buy today.

Using dd for Mirroring entire Partitions or Disks:

As demonstrated above, dd will do a job for you if your raid setup refuses to do a source -> mirror copy. Using modern hardware you should expect this to take a few hours.
[ using ancient hardware you will be there all day (literally) ]

If you prefer you can use dd for partition by partition mirroring, perhaps in my screenshot I might have used if=/dev/sda1 of=/dev/sdb1, so as to just copy the first partition.

Note: Doing things partition by partition, can be a bit more tricky, as you need to give thought to the partition structure on your output disk and if necessary create suitable partitions there prior to running dd.

Those who might prefer output to a file might like to consider of=/mnt/hugedisk/partition1.dd as a suitable parameter.

Documention for dd can be found in any of these places*:
  • For Debian and Ubuntu in /usr/share/doc/coreutils/
  • by typing in a terminal the command man dd
  • Manpage on the internet (
*Please make an effort to read the dd documentation above before making any comments on this article asking about how to use dd.

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