Re: accessing loopback filesystem+partitions on a file

[Luke Kenneth Casson Leighton <lkcl@lkcl.net>]

okay - i should be clearer.

the thing that is missing, that only xen provides, is the presentation
of another block device as a hard drive.

e.g. /dev/volumegroup/volumename --> /dev/loopblocka
or   /dev/loop0 -> /dev/loopblocka

such that it is possible to then subsequently do this:

fdisk /dev/loopblocka and
mkfs.ext2 /dev/loopblocka1
mount /dev/loopblocka1 -t ext2 /mnt/somewhere

you get the gist.

basically, the thing that is missing (or i can't find it)
from linux is a driver with the ability to present [any] block
devices with their major+minor numbers as a [fsck-]recogniseable
block device with its own major number, with the implicit
ability to create minor numbers within it.

lvm is in its own way a sort of mad-cap over-extended version
of the above, if you think about it carefully and can understand
the sentence.

l.


On Wed, Jun 29, 2005 at 02:35:25AM +0200, Grzegorz Kulewski wrote:
> On Wed, 29 Jun 2005, Luke Kenneth Casson Leighton wrote:
> 
> >[if you are happy to reply at all, please reply cc'd thank you.]
> >
> >hi,
> >
> >i'm really sorry to be bothering people on this list but i genuinely
> >don't what phrases to google for what i am looking for without getting
> >swamped by useless pages, which you will understand why when you see
> >the question, below.
> >
> >the question is, therefore:
> >
> >	* how the hell do you loopback mount (or lvm mount
> >	  or _anything_! something!)  partitions that have
> >	  been created in a loopback'd file!!!!
> >
> >	  [aside from booting up a second pre-installed xen
> >	  guest domain and making the filesystem-in-a-file
> >	  available as /dev/hdb of course.]
> >
> >answers of the form "work out where the partitions are, then use
> >hexedit to remove the first few blocks" will win no prizes here.
> 
> The bad news: it was impossible (or at least very hard to do).
> 
> The good news: it is possible now. The anwser is:
> - figure where the partitions are (possibly using some simple script),
> - use device-mapper to create block devices covering partitions,
> - mount them.
> 
> I do not know if this anwser will win your price but it is IMHO far better 
> than hexedit... :-) And probably this is the only anwser.
> 
> (IIRC if you have one partition you can skip partition table with offset 
> option to losetup. But this will only work in this special case...)
> 
> 
> Grzegorz Kulewski
> 

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