Re: [PLUG] /dev/loop/0 only once?

Jeff McAdams <jeffm@iglou.com> · 2007年6月04日 22:10:51 -0400

Jeff Abrahamson wrote:
> I was surprised to find that I can only mount one ISO image at a time:
> # mount -t iso9660 -o ro,loop=/dev/loop/0 debian-cd-1.iso debian-cd-1
> Try that again without an intervening umount and I'm told device
> busy. E.g., I want to look at 
> # mount -t iso9660 -o ro,loop=/dev/loop/0 debian-cd-2.iso debian-cd-2
> Sure enough, there's no /dev/loop/1, but why should I need it?
> I don't really need to do it, though it would be nice. But I'd really
> like to debug my understanding of mount. I thought this should work.
No, it won't. Basically, the loopback mount takes a filesystem image
(such as in the iso) and binds it to a device so that you can work with
it using Linux's device handling. So, think about it a second, when you
bind /dev/loop/0 to debian-cd-1.iso, that let's you deal with
/dev/loop/0 as a device with a filesystem to be mounted, the filesystem
is in debian-cd-1.iso, of course. When you issue the second command,
you're telling it to use /dev/loop/0 to access the filesystem in
debian-cd-2.iso...but its still being using to access the filesystem in
debian-cd-1.iso. So, when you do further operations...which file would
it do the operations in? So, you're asking it to do fundamentally
contradictory things.
What you should be able to do for a second command is
# mount -o ro /dev/loop/0 debian-cd-2
That will give you a second mount point to access the same filesystem,
but I doubt you'll find that very useful.
So, hopefully that helps you understand why you need /dev/loop/1 in that
case. With /dev/loop/1 in addition to /dev/loop/0 you can do what you
need. You can use the mknod command to create further /dev/loop/*
devices, they're block devices with major number 7, and minor numbers
starting at 0 and incrementing (typically).
-- 
Jeff McAdams
"They that can give up essential liberty to obtain a
little temporary safety deserve neither liberty nor safety."
 -- Benjamin Franklin