ISO Image dd To DVD+RW

[Frank Mitchell]

Hi:

Latest issue of Linux Format Mag gives you a DVD ISO Image for FreeBSD v8.0, 
which I happen to be interested in. I was also looking for an interesting use 
for my DVD+RW disks. I read somewhere you could "dd" straight from an ISO 
Image file to DVD+RW, though I couldn't remember where.

Previously I thought you could put an Ext2 Filesystem on DVD+RW, which turned 
out to be wrong. It worked the first time I tried, but not the second. But 
this experiment with "dd" worked, though I'm still unsure about how to use 
some of the options properly. Basically (under Linux) I put:

dd if=FreeBSD-8.iso of=/dev/sr1 ibs=512 obs=32768 conv=sync

And my DVD+RW wrote okay, showing:

3986344+0 Records In
62286+1 Records Out

Note that DVDs use 2048-byte Sectors like CDs, but they're handled in 
Error-Correcting Super-Blocks of 16. DVD+RW can write individual Sectors, but 
only by updating the entire Super-Block. My first attempt tried writing with a 
BlockSize of 2048, whereupon Drive /dev/sr1 sat there winding the disk back 
and forth, presumably writing every Super-Block 16 times over. With a total 
BlockSize of 32768 things went alot more smoothly. The 62286+1 Records Out 
are Super-Blocks with an extra one partially written. The original 3986344 
Records were 512-byte Disk Blocks, which make a whole number of 2048-byte DVD 
Sectors, but not Super-Blocks.

Then I wrote it back to a file so I could check the result using diff:

dd if=/dev/sr1 of=CHECK.ISO bs=2048 count=996586 conv=notrunc

Here it's okay to read the DVD as 2048-byte Sectors, because they're 
individually addressable. You need the count to stop the entire DVD getting 
imaged, all 4.7 GB, which happened the first time I tried. And of course it's 
the count of 2048-byte Sectors, which is 3986344/4.

CHECK.ISO was identical to the original Iso Image, and I was able to use the 
DVD+RW to install FreeBSD v8.0. Which left me with the same Xorg 
Configuration problems as FreeBSD v7.2, but that's another story...

Faictz Ce Que Vouldras: Frank Mitchell