dd copy of running system

MarchHare22 · 07-24-2009, 09:57 AM

Does anyone know if I use dd to copy a disk that is mounted and running to another that is mounted, it will cause problems?

GazL · 07-24-2009, 10:50 AM

Not recommended. If your source disk is written to while the copy is running then your destination copy will end up being inconsistent/corrupt.

Also, if the disks/partitions you're copying from/to aren't exactly the same size, you may also run into problems using dd in this way. Not to mention having to deal with duplicate UUIDS and what have you.

What is it that you're trying to do?

TSquaredF · 07-24-2009, 10:52 AM

I can't give you any hard/fast reasons why, but I have had problems trying to clone a mounted fs. Gazl's explanation sounds logical enough. I use rsync to copy partitions to a backup USB HDD, but have never been able to rsync a mounted partition. I use what I call a "recovery partition" (see my blog). You could also use a live CD.
Regards,
Bill

MarchHare22 · 07-24-2009, 11:00 AM

Well what I'm trying to do is copy a Linux reiserfs formatted disk to another, then I'm going to try to run off the copied disk. The problem is that I only have one system running, no way to run a live distro because it's on a mainframe. I have four disks I need to copy and then run the system from those copies.

Does anyone have suggestions? DD was really the only solution I saw to fit my needs.

GazL · 07-24-2009, 11:22 AM

Ahh I see. With your disks being mainframe dasd atleast they'll be the same.

You could try remounting the existing filesystems read-only while you do the copy. That should prevent the inconsistency problem.

Also, you might find some of the information on this helpful

http://www.linuxvm.org/Info/HOWTOs/lprclone.html

MarchHare22 · 07-24-2009, 11:28 AM

Haha that's exactly the tutorial I'm using! When I use DFDSS to copy the FS format is not recognized. And the first suggestion is under the assumption that I have two running systems. I only have one. I can put the root disk into read-only when I boot it? Is this a kernel parameter I can pass?

GazL · 07-24-2009, 11:32 AM

After you've got your Linux up and running you should be able to do a

mount -o remount,ro /dev/whatever /

MarchHare22 · 07-24-2009, 02:13 PM

I figured this out. You can't just copy over to an unformatted pack. It must be formatted for Linux, then copied. I figured that it would just raw copy over bit by bit and "become" formatted. I was wrong. So format your packs first people!

SavoTU · 07-24-2009, 03:36 PM

I have two identical hard drives and have used dd to copy everything from 1 to the other then pulled the old one out and used the install cd to fix lilo if needed.

I have always done this on a running system, i am not saying its a good idea but it worked for me.

Sav