Making Multi-Session CDs

Contents

## The Theory - Making Multi-Session CDs

NOTE: OpenBSD does not support creation of multi-session cds !!
If there is one procedure liable to test the user and his hardware, it is making multi-session CDs.
The potential is there in plenty for spoilt discs and prematurely fixated and closed CDs, if a carefull procedure is not followed.
The best way to avoid problems is to understand the process involved.
A multi-session CD is one with several tracks (in theory up to 99, but in practice you would be hard pushed to exceed 40ish) which were recorded separately, perhaps days or weeks apart.
Each track has a Table of Contents which links into the next free area of the disc, until the last track, which is closed.
This produces a contigeous virtual directory structure in which each track is seen as read from a common root directory.
HINT:- for this very reason, all data recorded should be sourced from one directory level above its root directory, or this virtual root directory will contain a whole lot of files from different tracks which are totally unrelated and difficult to separate.
Example:- I want to make a track containing all the config and setup files in the directory /etc.
If I specify the source path as /etc the fileimage will be fine as a stand-alone entity but when the resulting CD is viewed what you will find is all the files that were in /etc are in the CDs root directory and only the sub-directories from /etc such as /cron.d /cron.daily and so on are retained as separate directories.
If I copy /etc to /cd_mastering_area/etc and specify the source path of /cd_mastering_area the result is as was intended, /etc is grafted ONTO the root of the CD and not copied INTO its root
Quite simple - but you need to think about what you are doing

### The Practice - Steps to success - or otherwise

1. The first image should be made with multi-session checkbox off. Only use Rock Ridge extensions ie. Linux image type selected.
2. Once made, you can view this image through the View Image button, since it does not contain any references to data outside itself.
3. With the Single-Session radio button still enabled, check the Dummy Write checkbox and do a dummy write to test your media and the speed / buffer settings you have selected.
NOTE:-

Cdrecord reports SCSI errors if I try to do a dummy write with Multi-Session selected.
I cannot advise if this is a general problem or peculiar to my setup.
It could be related to apparant hardware problem my CD-Writer has with multi-session recording, requiring the -data switch be used with cdrecord.
See the MAN page for cdrecord which mentions problems with Sony devices regards multi-session writing and the -data switch on the Advanced Settings help page.

4. Next step is to set the Multi-Session radio button and unset the dummy checkbox and write the track.
If you have a refusal to write in Multi-Session mode, as happens on my HP CD Writer 9210i SCSI, I refer you back to the -data switch, it works for me.
Examination of the output from cdrecord should confirm a good write.
5. Each subsequent image requires a bit more work now.
mkhybrid needs to know the start and end addresses of the previous track on the CD and either the device address of the CD Writer which has the CD with the last written track, or the full path of the last image file.
Selecting the Multi-Session radio button has enabled 3 edit boxes below it.
The 3rd box should be pre-filled with the device details taken from your initial selection of a CD Writer.
Boxes 1 and 2 can be filled for you by pressing the [Fetch Track Info] button above the edit boxes. This runs cdrecord with the -msinfo switch and parses the output into the appropriate box.
Running Make Image for the second set of data produces an image which mkhybrid reports as being far larger than it is. This is because the size is the cumulative total of the 2 images so far made.
It proves that the linkage between them has been made and also gives you a good idea of the overall data size of the CD so far.
6. This image will not mount via the View Image button. mount reports every possible fault if you try it for the reasons previously discussed.
NOTE:-

If you want to view and check the file image for a given set of data you will have to follow a different course.
Change the name of the output file (I normally put an 'a' on the end eg image2a.raw) and then amend the Track End value to zero, whilst retaining the Track Start figure and Device as previous.
The resulting image is mountable through the [View Image] button in CD-Rchive. It contains references to all the previous images linked to it, although attempting to access the data from these other images would give an I/O error.
You might want to carry out this procedure once as a confidence boosting measure, but it is hardly something you would do after each image make. Besides anything else, the fact that you can mount a different image does not absolutely guarantee that you will successfully write the first one.

7. Write the image to CD and repeat as required ( Steps 5 & 7) until the last track
8. Make the image the same way but select Single-Session radio button prior to writing. Thus the last track will be written without links to the next free address on the CD, properly fixated and closed.
No more data can now be added.

### Usage

It might be tempting to use multi-session CDs as a form of incremental backup, adding tracks until the CD is full and closing it.
I have included the ability to do just that, so the choice is yours.
The theory is fine, something like Adaptec DirectCD under Windows. The practice is that one error on your part, one lost packet in a write when a cron you forgot was scheduled suddenly kicked in and re-indexed your document server, and you could lose access to all or part of everything on the CD.
Much safer to master the data on hard disk until you have a CD full and then write in one operation, test the CD, compare its contents to the master THEN delete the master if required.
I would advise you save multi-session CDs for photo CDs or multi media as they were intended. But what do I know?!
Recording on the Fly