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Re: dumb question about mounting iso file as loop device....

* Michael Fothergill <mikef20000@hotmail.com> [061218 22:00]:
> In the jigdo how to manual it says that you can update an image with jigdo 
> either by scanning in the image from a CD or DVD
> OR
> if you have an ISO file on your hard drive you can mount it as a loop 
> device:
> Here is some material from the how to on this:
> On the other hand, if you have an ISO file you'd like to update, mount it 
> as a loop device (you may need to be root to do this). Using Woody as an 
> example:
>      # mount -o loop woody-i386-1.iso /mnt
> What does this mean?
> Do you have to move the iso file to the /mnt directory and then run this 
> mount command?

When you run the mount command, the command must be able to find the
ISO image.  So if the ISO image is in "/home/mf/debian-iso-images/dvd"
then either execute:

    # cd /home/mf/debian-iso-images/dvd

    # mount -o loop woody-i386-1.iso /mnt

or else, from any directory, execute:

    # mount -o loop /home/mf/debian-iso-images/dvd/woody-i386-1.iso /mnt

> What is the idea of a loop device?

The concept?  That's what they make Google for; search a bit.  While
you're at it, you need to search for "mount point". 

But as to what the loop option accomplishes: Mounting an ISO file with
the loop option allows the file to be read as if it were a CD or DVD.

> Also what I want to do is to take the iso file for the DVD image of Etch 
> and make CD images from it.....
> I have both DVD that I burned of this image and the original iso file I 
> downloaded with the help of jigdo.
> It seems to take ages for the DVD to be scanned by jigdo before it can get 
> around to making the CD iso.....
> Would it be faster using the iso file on the hard drive together the loop 
> command?
> I don't need to update the DVD iso itself just make the CD images from it?

FIRST.  To "update" an ISO image means to create a new ISO image after
first scavenging from the old image all the files which still are
current.  The original ISO file itself is not overwritten (unless it
happens to be in the directory in which you are running jigdo AND it
happens to have the same name as the new ISO image, in which case the
old image is destroyed).  Thus, "updating" is something of a misnomer.

SECOND.  An ISO image on the hard drive, mounted as a loopback device
(option = loop) can be read faster than a DVD can be read.

THIRD.  There are tradeoffs.  I recommend that you keep on hand an
up-to-date set of DVD ISO images, because it is simpler to update
three DVD images than it is to update fifteen CD images.  The tradeoff
comes when you wish to create CD images from the DVD, because in the
worst case it is necessary to scan the entire DVD (which contains as
much data as do five CDs) in order to glean the files for each CD.
But I assume that you update the DVD images weekly, and that you have
need for CDs only infrequently.


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