On Tue 06 May 2008 07:49:01 NZST +1200, Carlos E. R. wrote:
Because without it some padding bytes are read and the md5sum will be different. That is the explanation I read and the personal experience as
Does that apply to the md5sum command used directly, or only via dd?
There are several issues which cause the equivalent of a cat /dev/sr0 to not return the correct number of bytes, making it useless for md5 sums. The only functional way to verify checksums of burnt media is to obtain the number of blocks occupied by the iso9660 filesystem, e.g. using isoinfo -d -i, and then to use dd bs=2k count=thatnumber - and hope the kernel doesn't screw you. The kernel detects EOM (end of media) by attempting to always read another bunch of 2k blocks. This fails to give the number of blocks you want/need because the burning process tends to write some blocks full of zeros at the end. Worse, a kernel problem present since the mid/late 90s attempts to read more blocks then there are, killing the lot with the ensuing I/O error, so you never get the last blocks used by the ISO filesystem. While mucking around doing this the drive in question (and all other accesses to it, including the other drive on the same IDE cable) is deadlocked for quite some time. Things are worse when DMA is enabled. Relief can be obtained by disabling read-ahead with hdparm -a0 at the expense of serious(!) performance loss. Not sure whether sdparm has a similar option (if not you're stuffed). dd bs=2k count=1 skip=N will never save you as the read-ahead will always read a chunk of blocks at once. For this reason I always burn with a script which chucks a pile of 0-filled blocks at the end of everything. If you install my scriptutils package the commands md5 -I / -y and writecd --blockread will always read the correct number of blocks for checksumming etc - as long as there is no I/O error. HTH, Volker -- Volker Kuhlmann is list0570 with the domain in header http://volker.dnsalias.net/ Please do not CC list postings to me. --------------------------------------------------------------------- To unsubscribe, e-mail: opensuse-factory+unsubscribe@opensuse.org For additional commands, e-mail: opensuse-factory+help@opensuse.org