Xen devel - Jun 2005 - accessing loopback filesystem+partitions on a file

[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.

background:

	i''m sort-of helping test a xen install project where a
	block device is presented as the DRIVE - not, i repeat
	not, the partitions on the drive which is the quotes
	normal quotes way of doing xen installes

	(yes there are good reasons for doing this).

	the thing is that the install is failing, and we''d duh
	like to analyse what''s going on (access the log files -
	no, sshd hasn''t been installed yet) after terminating
	the xen session [because afaik in xen guest sessions
	there''s no way i know of to access virtual consoles 2-6]

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.

l.

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel

Luke Kenneth Casson Leighton wrote:
>[if you are happy to reply at all, please reply cc''d thank you.]
>
>hi,
>  
>
Hi Luke,
>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!!!!
>  
>
See http://lists.samba.org/archive/linux/2004-December/012627.html

In summary, if you have a single partition then you can use the 
following command:

mount -o loop,offset=32256 loop.img /mnt

There''s mention of a replacement loopback driver that supports 
partitions but a quick google''ing didn''t turn anything up.

Regards,

Anthony Liguori

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel

> 	i''m sort-of helping test a xen install project where a
> 	block device is presented as the DRIVE - not, i repeat
> 	not, the partitions on the drive which is the quotes
> 	normal quotes way of doing xen installes
>
> 	(yes there are good reasons for doing this).
That''s fine.  It''s something that (is supposed to) works.
> 	* how the hell do you loopback mount (or lvm mount
> 	  or _anything_! something!)  partitions that have
> 	  been created in a loopback''d file!!!!

You could work out where the partitions are, then use hexedit... woops!  just 
kidding ;-)

This has come up before but it''s easier to find the list entries when
you
remember what you''re looking for ;-)

There''s a patch by NASA (of all people!) that enables the Linux
loopback
driver to support partitions:
ftp://ftp.hq.nasa.gov/pub/ig/ccd/enhanced_loopback/readme.txt

There''s also the "lomount" utility from QEmu:
http://www.dad-answers.com/qemu/utilities/QEMU-HD-Mounter/lomount/
Which works out where partitions are and mounts them (no kernel mods needed).

Apparently kpartx (from the multipath tools - 
http://christophe.varoqui.free.fr/wiki/wakka.php?wiki=Home) can use the 
device mapper to create block devices for each partition.

I think I''d go with lomount, as it looks easiest.

Cheers,
Mark
> 	  [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.
>
> l.
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel
_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel

EXCELLENT.  thank you.

On Wed, Jun 29, 2005 at 02:33:05AM +0100, Mark Williamson
wrote:
> > 	i''m sort-of helping test a xen install project where a
> > 	block device is presented as the DRIVE - not, i repeat
> > 	not, the partitions on the drive which is the quotes
> > 	normal quotes way of doing xen installes
> >
> > 	(yes there are good reasons for doing this).
> 
> That''s fine.  It''s something that (is supposed to) works.
 
 yep.  it does.
> > 	* how the hell do you loopback mount (or lvm mount
> > 	  or _anything_! something!)  partitions that have
> > 	  been created in a loopback''d file!!!!
> 
> 
> You could work out where the partitions are, then use hexedit... woops! 
just
> kidding ;-)
> 
> This has come up before but it''s easier to find the list entries
when you
> remember what you''re looking for ;-)
> 
> There''s a patch by NASA (of all people!) that enables the Linux
loopback
> driver to support partitions:
> ftp://ftp.hq.nasa.gov/pub/ig/ccd/enhanced_loopback/readme.txt
 *curious* - why haven''t these mods been merged back yet?
> There''s also the "lomount" utility from QEmu:
> http://www.dad-answers.com/qemu/utilities/QEMU-HD-Mounter/lomount/
> Which works out where partitions are and mounts them (no kernel mods
needed).
 _great_.
> Apparently kpartx (from the multipath tools - 
> http://christophe.varoqui.free.fr/wiki/wakka.php?wiki=Home) can use the 
> device mapper to create block devices for each partition.
> 
> I think I''d go with lomount, as it looks easiest.
 
 okay.

 question for you.

 would there be any reason (e.g. performance increase)
 which could justify providing the block device remapping
 facility at the linux device driver level instead of in
 xen''s fun-and-games?

 the non-technical (but aesthetic) justification is that
 xen provides the facility to represent block devices
 (major+minor) for use as generic block devices _anyway_ -
 just in a rather roundabout (and exclusionary i.e.
 "unfair!! i wanna play!!") manner.

 l.

 p.s. mark, what the hell are _you_ doing up at 3am??? :)


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel

On Wed, Jun 29, 2005 at 02:33:05AM +0100, Mark Williamson wrote:
> Apparently kpartx (from the multipath tools - 
> http://christophe.varoqui.free.fr/wiki/wakka.php?wiki=Home) can use the 
> device mapper to create block devices for each partition.
 ah HA.

 well, given that phil has modified his d-i xen stuff to utilise lvm
 partitions (as the disk(!)) because i told him how abominably slow
 loopback is, this might actually turn out to be the best way to go.

 muhahahah.

 l.

 p.s. http://hands.com/d-i.  not officially announced.  don''t blame me
if
 it all goes horribly wrong.  search for "xen0".  another way to
install
 xen guest OSes that does NOT involve using debootstrap.


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel

On Wed, Jun 29, 2005 at 11:48:40AM +1000, Peter Chubb wrote:
> Luke> that loopback filesystems cannot be presented as block devices
> Luke> by the linux kernel (with no involvement of xen) seems to be a
> Luke> curious omission.
> 
> But they can!  But a loopback device can''t be partitioned.  So do
it
> one partition at a time.
 ian and mark kindly responded pointing out some programs which
 make that possible.

 i liked the LVM one best.
> You''ll probably only have a few real filesystems on the disk image
> anyway.
 
 hi peter, thank you for responding.

 see http://hands.com/d-i, searching for "xen0".

 the key is to be able to test-run debian installations in
 a xen guest domain (with absolute minimal changes to the
 packages or the debian boot installs).

 and of course a debian install expects to see a hard drive, which it
 expects to be able to partition - so it is given one.

 l.


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel

On Tue, Jun 28, 2005 at 07:30:58PM -0500, Anthony Liguori
wrote:
> Luke Kenneth Casson Leighton wrote:
> 
> >[if you are happy to reply at all, please reply cc''d thank
you.]
> >
> >hi,
> > 
> >
> Hi Luke,
> 
> >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!!!!
> > 
> >
> See http://lists.samba.org/archive/linux/2004-December/012627.html
> 
> In summary, if you have a single partition then you can use the 
> following command:
> 
> mount -o loop,offset=32256 loop.img /mnt
> 
> There''s mention of a replacement loopback driver that supports 
> partitions but a quick google''ing didn''t turn anything
up.
 
 yeh, i know - pain, innit - hence my question!

 you''d have to know in advance that someone worked on this
 already such that they picked a suitable keyword e.g. "extended
 loopback" or "enhanced loopback".

 the latter gives this:

	 http://unix.freshmeat.net/projects/enloop/

 fortunately the knowledge is stored in a number of people''s heads,
 namely ian and mark''s, and they''ve kindly responded and
pointed me in
 a suitable direction.

 lomount.c, a 259-line-program which cannot be found anywhere
 but in source code form, actually analyses the first block
 and performs above-suggested suitable mount commands on your behalf.

 l.


_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xensource.com
http://lists.xensource.com/xen-devel