Esteemed gurus, if anyone is able to shed some more light on this kernel problem I'd appreciate it. It's been going on for years.
When reading the whole of an iso9660 filesystem from a cd or dvd, I often get I/O errors within the last blocks of the filesystem. Using cat /dev/cdrom as test is not a good idea, as the kernel often has problems detecting the end of the recording on the media (however it does that). Also, reading additional blocks past the end of the filesystem makes it impossible to use md5 sums. A better way is to obtain the number of blocks used by the filesystem and reading exactly that many blocks: isoinfo -d -i /dev/cdrom dd if=/dev/cdrom bs=2k count=X (I scripted that a long time ago). In the mid nineties it was necessary to use cdrecord -pad to make linux read all the files, described as read-ahead bug in linux's iso9660 driver in the cdrecord man page. When I checked again a few years ago it was still necessary to use -pad, so I kept on always using it as a matter of course. This year I find that cdrecord -pad is no longer enough, and the same problem shows up again. It affects cds and dvds, and all pressed commercial cds cerated on non-linux systems. The problem shows up at varying degrees, I suspect it has to do with the modulus of the number of filesystem blocks of the particular disk. Several times I was unable to completely read the filesystem from cds, I/O error 2 or more blocks beore its end no matter what I did. Often it helps to use dd skip= to only read the last 10 filesystem blocks, and it works. Reading all filesystem blocks from the beginning, or from a suitably large distance from the end - I/O error. Sometimes the problem goes away when turning DMA off for the cd/dvdrom drive, but that is not an acceptable way of running it. I really doubt it's hardwaredependent, as I have seen it on 3 totally different machines now. Kernels 2.4.19, 2.4.20. It doesn't help that read problems on a cd/dvd can cause the IDE bus, if not the system, to lock up for some time. Appending 2MB of zeroes to the disk image file before burning makes it disappear reliably. I keep on wasting CDs because I forget to append that dummy data. The issue could be explained with the kernel always reading whole chunks, and that the chunk size was increased somewhere around 2.4.19 with the effect that cdrecord -pad (writing an extra 15 blocks = 30kb) is no longer sufficient to ensure the last chunk is readable. This is speculation however. This state of affairs is not really acceptable. Does anyone know what the problem is caused by, and what can/should be done about it? Thanks much, Volker -- Volker Kuhlmann is possibly list0570 with the domain in header http://volker.dnsalias.net/ Please do not CC list postings to me. -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]
