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I'm currently tasked with bringing up a existing server to (at least more) professional standards - with as little disruption as possible to end-users. To have a proper rollback, I need to backup the existing server's root fs, which is currently ext4 inside a primary disk partition - not LVM, which would make the whole show a mere finger excercise. I'd like to avoid shutting down the server.

Is it even possible under these circumstances?

2 Answers 2

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You don't need LVM. LVM just uses the device mapper, too. You can do that yourself. That may be a bit tricky with the root fs, though, as you probably have to modify the boot scripts, as the following won't work when the device is mounted.

You just need to put a DM device on top of the root device, e.g.

sectors="$(blockdev --getsz /dev/sda3)"
dmsetup create rootfs --table "0 ${sectors} linear /dev/sda3 0"

Something like this has to be put into the initrd boot scripts. Instead of /dev/sda3 you would then mount /dev/mapper/rootfs to /. Then you can make snapshots of the root fs (but have to do all the steps by hand with dmsetup; no black magic though). You just can't merge them back but that should not be a problem.

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  • That is quite elegant. I'll read into that. Thanks.
    – Roman
    Feb 22, 2013 at 13:53
  • If you manage to get your / mounted from /dev/mapper/rootfs (what IMHO every distro should do) then I can provide the code for snapshopt creation. I already do that: My LVs are exported to a KVM guest and appear there as regular disks. Thus I have to put DM devices on top of them. Not a big deal with non-root fs, though. Feb 22, 2013 at 13:59
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    Could you also post the snapshot script please?
    – user128063
    May 5, 2017 at 13:54
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You can try BMR solution provided by bacula

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  • 1
    No, I can't. This is for a completely different purpose.
    – Roman
    Feb 22, 2013 at 13:50

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