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I'm trying to back up an LVM volume using Duplicity (note: not just the file contents of the volume, but the actual volume itself, byte-for-byte).

The best way I've so far found is to dd all the data on the LVM volume to a temporary file, include that file in the backup set, and delete it after the backup is made. This seems horribly inefficient, though: I'm needlessly thrashing the disk and duplicating all the data.

What would seem to be needed is a way to do essentially the opposite of mount -o loop. In other words, I don't want to mount a file as a block device: instead, I want to 'mount' a block device as a regular (read-only) binary file — one that Duplicity will then read from and back up for me.

Is there any way to achieve this?

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Sure you can, remember that in Unix "everything is a file" (almost). I don't know Duplicity, but just including the relevant /dev/mapper/... file might be enough. But this will copy everything, including spaces never used and remnants of deleted files. To back that up only makes sense for forensic purposes. –  vonbrand Jan 29 '13 at 16:20
Hi @vonbrand — yes, that's what I was hoping too. So perhaps Duplicity has built-in special treatment for block devices which is screwing this use-case up. –  George Jan 29 '13 at 16:59

2 Answers 2

That's how block devices already work by default. They can be read by any program – after all, dd uses just the standard open() and read() functions to create the image.

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Hmm — it definitely doesn't work with Duplicity. Perhaps the problem is that Duplicity has built-in special treatment for block devices. –  George Jan 29 '13 at 16:09

This is the normal way things already work.

Just add -r or -o ro to the mount options to make it read-only.

Man mount

 ro      The same as -r; mount the file system read-only 
         (even the super-user may not write it).
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