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Old 02-07-2007, 01:32 AM   #1
wahming
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dd fresh install to separate partition


I'm tired of reinstalling linux on my sandbox pc. Would it be feasible to create a /boot partition, create 2 more equal-sized partitions, use one of them as /, and use dd to backup the / partition to the other? Upon screwing up /, I could then boot to the backup and restore / to the fresh install. Am I missing some other factor?

Edit - anybody who got confused by the convoluted post above, raise your hands.

Last edited by wahming; 02-07-2007 at 01:54 AM.
 
Old 02-07-2007, 02:43 AM   #2
SciYro
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it should be possible, and it should work, but only if the partition sizes match perfectly. If this isent possible, another solution is to tarball the entire / directory tree, store it somewhere safe, and when you need to recover, boot in, reformat /, then unpack the tarball, then run "lilo" to reset the bootloader (if you use another bootloader, just make sure it will boot what you unpacked).
 
Old 02-07-2007, 02:49 AM   #3
wahming
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Trying it out right now. At least I have something to try the next time I crash the sandbox.

Btw, dd if=/dev/hda2 of=/dev/hda3 isn't going to screw up the partition table, is it? Not sure if the EBR's are actually located on the drives themselves.
 
Old 02-07-2007, 03:04 AM   #4
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If both drives are similar models and brands, using dd could work. To do it correctly, I suggest the following.
dd if=/dev/hda of=/dev/hdc conv=sync,notrunc bs=512

This will do a sector by sector copy. Though it will not work if the drives are different. I recommend using dar to create mirror copy of your files. Unfortunately, you will have to create the partitions and mount them on your own.
 
Old 02-07-2007, 03:10 AM   #5
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I believe the partition table is located within the boot sector, in any case, i dont think Linux exports that info thru /dev/hda*, /dev/hda and you can mess with partition tables
 
Old 02-07-2007, 03:12 AM   #6
wahming
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Right, thanks for the help
 
Old 02-07-2007, 08:31 AM   #7
pixellany
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Quote:
Originally Posted by SciYro
I believe the partition table is located within the boot sector, in any case, i dont think Linux exports that info thru /dev/hda*, /dev/hda and you can mess with partition tables
I think it's like this: The mbr--which includes the **primary** partition table-- is the first sector of the drive. Then each partition has a "boot sector" which can contain more boot code + the partition tables for logical partitions.
 
Old 02-07-2007, 08:33 AM   #8
wahming
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I got that much. I just wanted to know if the dd command would copy the "partition boot sector" as well. I gather not, from SciYro's response.
 
Old 02-07-2007, 08:51 AM   #9
pixellany
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Just when you think you understand things.....My system has 63-sector gaps between all the logical partitions, but no gaps between the primaries.

So if I use dd to copy a primary, I get the boot sector, but not if copying a logical????

Someone bring me back my Apple-II....
 
Old 02-07-2007, 08:57 AM   #10
wahming
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Well, you could copy the extended partition - that'd give you all your boot sectors minus the three primary ones =D

Also a whole ton of data/junk, oh well
 
Old 02-07-2007, 12:16 PM   #11
pixellany
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Quote:
Originally Posted by wahming
Well, you could copy the extended partition - that'd give you all your boot sectors minus the three primary ones =D

Also a whole ton of data/junk, oh well
Does anyone know if this actually works? You can't normally access an extended--eg you cannot mount it.
 
Old 02-07-2007, 12:22 PM   #12
pixellany
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Quote:
Originally Posted by pixellany
Just when you think you understand things.....My system has 63-sector gaps between all the logical partitions, but no gaps between the primaries.

So if I use dd to copy a primary, I get the boot sector, but not if copying a logical????

Someone bring me back my Apple-II....

OK!!! partial answer

At the front of the disk are 63 sectors (before the 1st primary partition)
The first sector is the mbr and has the stage1 boot code + the primary partition table.

Then--in front of every logical partition, there is another 63 sectors, with the first one containing a two entry partition table:
the first one points to the logical partition (63 sectors down)
the second one points to the next 63-sector segment preceding the next logical

If there is boot code in a partition, it appears to go inside the actual partition, and not in the 63-sector dividers.

So--yes--copying a partition will capture the partition boot sector (but not the mbr)
 
Old 02-07-2007, 09:00 PM   #13
wahming
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The dd process I suggested, "dd if=/dev/hda2 of=/dev/hda3" appears to be a bad idea. Linux now refuses to boot without moaning about filesystem errors and what not. hda3 is listed in fstab as LABEL=/backup, but dd has apparently changed its label to LABEL=/. This has resulted in TWO root filesystems (far as Linux is concerned) causing it to panic and reboot at startup.

Solution - enter rescue mode, edit /etc/fstab and /boot/grub.conf to look for /dev/hda2 instead of LABEL=/.
 
Old 02-08-2007, 09:04 PM   #14
wahming
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Update- the restore process worked like a charm. 1 thing - I had to boot into the restore partition (hda3) before I could overwrite hda2. Skipping that step led to a corrupted hda2.
 
  


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