From mkisofs man page, 2.01:
What is the difference between a second session and a higher level one? Thanks in advance.

There is no difference -- all of them are later than the first session.

For example you'd like to burn the contents of the current directory and prepare CD-ROM disk for the burning of the future sessions. In such case you can prepare image.iso file using the command:

and burn it in multisession mode using the command:

After that you could burn the contents of the other directory on the same disk. In such case you can prepare image.iso file with the command:

and burn it in multisession mode using the same command as before:

The resulting CD-ROM disk has data stored in both burned directories. If some file burned during the consecutive session has the same name as the file burned previously the newer file overwrites the older one.

To close CD-ROM disk write the closing session with the command:

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OK, and thanks a lot. But is the space ocupied in that disk by both of the directories the same as the sum of the space occupied by the first on some disk and the second burned on some other disk?

Assuming you change the contents of some file and you burn it for a few times on the same CD-ROM disk each consecutive burning occupies more of the disk space because all previous burned sessions persist on that disk. As for different files burned in a few sessions I suppose they occupy slightly more CD-ROM disk space than the same files burned in one session.

I suggest you to perform simple experiment. Collect ten files of about 500 MB in total and burn them in one session on the first CD-ROM. Then burn the same files in ten sessions on the second CD-ROM. Use the CD-ROM disks by the same manufacturer and of the same type in both cases. After that compare the sizes of both CD-ROM disks.

Finally publish the results of that test here for other people knowledge.

Kind of you to have replied. I'll put to work on what you've suggested. But, I anticipate there will be a difference within 5% in used CD-ROM space. The alternative would be like this (example): file A= 100M, file B=150M. First session: I record file A. Second session: I record file B. Now, used space on CD-ROM= 100M from first session + (100M + 150M) from second session. That is, file A has been written twice on the disk, consecutively. And my question was: is this the case? Regards.

I tried to burn two sessions on one CD-ROM disk but I failed for the three times. My three CD-ROM drives refused to work. I'm too tired now to test that throughout. I still have a few empty CD-ROM disks so maybe I'll try to write them tomorrow.

As for your question I suppose you burned during the second session both A and B files.

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No, I only burned file B duriong the second session. And this answers my question in the negative. Ie, there is no redundancy and file B is, on the final disk, only figuring once. Thanks a lot.