Re: accessing loopback filesystem+partitions on a file

Luke Kenneth Casson Leighton
2005-06-29T01:43:05Z

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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