An explanation of UNIONFS?

Would someone be able to give me a run down of what UNIONFS is and what it does?
I understand that it's a filesystem allowing write-only and read-write branches, but how is it used with Knoppix 3.8? Will it be a way of creating space on a hard disk allowing Knoppix to read and write to the disk, while keeping the CD read-only, or am I missing the point? I'm a little confused as you can probably tell :p

Thanks,
Mark

I have a short writeup of what UnionFS means for Knoppix here:

http://www.oreillynet.com/pub/wlg/6654

I don't go into it in explicit detail, but you can read the official UnionFS project page for that level of info anyway.

Ok, I think I get it now, just confirm if I'm right if you would:

<=3.7, you could edit /home and use the Save Configuration utitlity to save the settings and files to a disk, e.g. a USB drive or a hard disk.

With UNIONFS, Knoppix is completely read/write, meaning you can install new programs on the /ramdisk, then save/restore in the same way you could with /home on <=3.7 (I understand it's slightly mroe complex than that, but I see that as the basics)

Is that right?

Thanks,
Mark

Well in fact, you don't even have to think about the /ramdisk yourself, you just write directly to /usr/ or whatever directory you want, and UnionFS figures it all out for you. Then you run the persistent script so that the changes are saved.

I am running 3.8 from an iso image on the harddisk and swap on the harddisk however persistent homedir is not enabled.

According to mount, /UNIONFS is the union of /ramdisk and /KNOPPIX

According to df, the size of /UNIONFS is 6.1G - but /ramdisk is 391M and /KNOPPIX is 1.9G - so where is the other 4G coming from?

Code:

---

knoppix@1[tmp]$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/root            3.4M  23K  3.4M  1% /
/dev/hda1              28G  4.7G  22G  18% /cdrom2.loop
/cdrom2.loop/knx38.iso
                      695M  695M    0 100% /cdrom/
/dev/cloop            1.9G  1.9G    0 100% /KNOPPIX
/ramdisk              391M  391M    0 100% /ramdisk
/UNIONFS              6.1G  6.1G    0 100% /UNIONFS

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Also, when I wrote the large file to /ramdisk that filled it completely, there was considerable disk activity. I am wondering what on the harddisk it would be writing to.

How much swap do you have? That could explain the disk activity.

Mark