Comment CDRW Info (Score 1) 11
The major issues with CDRWs (above and beyond UDF write support) are:
1. CDRW drives don't do bad block remapping. So all bad block remapping has to be handled by the filesystem (UDF supports this). Of course, the drive also doesn't tell you when the write failed. So you have to read back the block to see if the write succeeded.... BUT
2. You can't just write blocks to a CDRW (well, ok.. you can.. BUT.. writing to CDRWs consists of writing packets.. now, your packet length could be 1 (in which case, your writing blocks to the media) but for each packet, you incure a 7 block overhead (4 blocks lead-in, 3 blocks lead-out or something along those lines). So, take 650 MB, divide by 8, and you get the size of the disk written with 1 block packets. Not very useful :)
3. Soo.. The packet size used is 32. Which means, when you write to the media, you have to write 32 blocks, or 64k.. Of course, as far as I know, the kernel has no support for doing this (the scsi generic module doesn't even allocate a large enough buffer to support it in userspace).
4. But remember, you gotta check if all the blocks in the packet were successfully written.. So after writing the 64k chunk, you have to read it back.. Fun!
5. And last but not least, unless you have a really new CD drive (and even then) you might not be able to read a fixed packet written disk in anything but your CDRW. If your looking for a new CD drive, you want a drive that handles both fixed and variable packet writen media.
I think I got all the gotchas for CDRWs. At least, as far as my knowledge goes. If I said something totally wrong, let me know :)
1. CDRW drives don't do bad block remapping. So all bad block remapping has to be handled by the filesystem (UDF supports this). Of course, the drive also doesn't tell you when the write failed. So you have to read back the block to see if the write succeeded.... BUT
2. You can't just write blocks to a CDRW (well, ok.. you can.. BUT.. writing to CDRWs consists of writing packets.. now, your packet length could be 1 (in which case, your writing blocks to the media) but for each packet, you incure a 7 block overhead (4 blocks lead-in, 3 blocks lead-out or something along those lines). So, take 650 MB, divide by 8, and you get the size of the disk written with 1 block packets. Not very useful
3. Soo.. The packet size used is 32. Which means, when you write to the media, you have to write 32 blocks, or 64k.. Of course, as far as I know, the kernel has no support for doing this (the scsi generic module doesn't even allocate a large enough buffer to support it in userspace).
4. But remember, you gotta check if all the blocks in the packet were successfully written.. So after writing the 64k chunk, you have to read it back.. Fun!
5. And last but not least, unless you have a really new CD drive (and even then) you might not be able to read a fixed packet written disk in anything but your CDRW. If your looking for a new CD drive, you want a drive that handles both fixed and variable packet writen media.
I think I got all the gotchas for CDRWs. At least, as far as my knowledge goes. If I said something totally wrong, let me know
