dd fails to read DVD until after it has been played in VLC

Sipos

I'm encountering some strange behaviour from dd that, while it isn't really causing me much of a problem, I am puzzled as to why it happens.

I am making an image of a DVD video, on my hard disk, using dd (I'm running dd if=/dev/sr0 of=dvdImage.iso). If I try this for any disk I have just inserted, it fails with the following error:

mingdao · n00b Joined: 19 Sep 2011 Posts: 10 Location: Alabama, USA

FWIW my system works fine with that command. This is the output
after running it for ~10 seconds then stopping with "Ctrl+C":

NeddySeagoon · Posted: Mon Sep 19, 2011 6:12 pm Post subject:

Sipos,

Many dvds cannot be copied with dd nor by mounting the filesystem and copying the files.
The problem is that DVDs are made with deliberately corrupt data blocks, so you get I/O errors.

These deliberately corrupt data blocks do not interfere with DVD players and your enjoyment of the film as they are never read by DVD players.
Instead, DVDs are played by reference to a block list.

A DVD player operates by reading a list of blocks on the DVD which provides the list of blocks to be played and the order to play them in.
Trying to make an image of a DVD is a lost cause. This is why DVDs are not normally mounted to play the video - the file system is not used.

Gusar · Posted: Mon Sep 19, 2011 6:30 pm Post subject:

That command often won't work, you need to use dvdnav://1

The reason is intentionally physically corrupt sectors on the disk. The parts of the disc which contain dvdnav info also contain a list of which sectors those are, so a player can skip them.

Maybe using ddrescue instead of dd would work, but there's no guarantee such an image will work correctly. You can give it a try though, and report.

NeddySeagoon · Posted: Mon Sep 19, 2011 7:33 pm Post subject:

Gusar,

ddrescue will be very slow and never terminate as it can't read the corrupt sectors.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

Gusar · Posted: Mon Sep 19, 2011 7:41 pm Post subject:

Shouldn't ddrescue skip the corrupt sectors and put zeros in that place in the resulting image?

That's the problem we have now - apps, including mplayer, hang while trying to read a corrupt sector. If ddrescue can't get around that, what benefit does it bring then over the plain dd?

Sipos · Posted: Mon Sep 19, 2011 9:14 pm Post subject:

I think that the discs I was asking about don't have intentionally bad sectors. As I said, I can copy them fine with dd after I have played them once. There are no errors and the resulting images seem to work fine. I can also mount them, copy the files with cp and play them. If they had some bad sectors, surely dd and cp should still fail to copy them, whether or not I played them once with VLC first? How would playing them with VLC affect dd? This is what I am confused about.

As it happens, I do have another DVD that does seem to have intentionally bad sectors and, your replies have helped me dump a stream from that using

NeddySeagoon · Posted: Mon Sep 19, 2011 9:20 pm Post subject:

Gusar,

ddrescue will only exit when it has read the entire filesystem without errors. It does not so much ship the corrupt blocks as come back to them later.
It tries very hard to read defective blocks, by stepping the read head in from both sides, by reading bigger/smaller 'chunks' and always fails as the sectors are corrupt be design.

You need to stop it manually.

mplayer (or any other DVD player) will never attempt to read the corrupt blocks as it uses the same approach that all DVD players do to avoid them.
It fetches the useful data only by following the block list(s).
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

NeddySeagoon · Posted: Mon Sep 19, 2011 9:27 pm Post subject:

Sipos,

DVD drives come in two flavours. Old ones are RPC1 recent ones (the last 10 years) are RPC2.

A RPC1 drive will always give raw access to the DVD. I still have one of those somewhere.
A RPC2 drive will only allow raw access after the region code has been verified.

cp and dd will not carry out region code negiotiation but a DVD player will.
So vlc or whatever, does the region code check then the drive allows dd raw block level access.

Note that if you have a region code mismatch between the drive and DVD, you won't get raw access and cannot play the video.
Thats the theory anyway.
_________________
Regards,

NeddySeagoon

Computer users fall into two groups:-
those that do backups
those that have never had a hard drive fail.

Sipos · Posted: Mon Sep 19, 2011 9:38 pm Post subject:

Sipos · Posted: Mon Sep 19, 2011 9:45 pm Post subject:

Sipos · Posted: Mon Oct 10, 2011 8:37 pm Post subject:

Do either of you know of a program I can use that will just make sure that the region lock stuff is done and a disc is readable properly?

Obviously, I can try playing in mplayer but, ideally, I'd like something that will exit with a non-zero exit code only if there is a problem with the region (for example if I used a disc with a region not supported by my drive).