Problems with reading DVD using 2.6.21.5

Problems with reading DVD using 2.6.21.5

[Manuel Reimer]

Hello,

today I've tried to install Slackware 12.0

As the installer just "skipped" some install steps, I tried to find the 
error.

The problem seems to be unreadable parts on the DVD:

http://pastebin.com/f381e8a88

But the DVD is OK. I've checked the MD5sum directly from disc on the 
same system using the same DVD drive.

dmesg says:

http://pastebin.com/f63c5c389

The kernel, used on the Slackware setup disk, uses SMP, but my hardware 
doesn't support this (get error on dmesg). May this (SMP kernel on 
non-SMP system) cause such bugs?

Is this a known bug? How could code, which breaks DVD access, get into 
stable 2.6.21.5?

Thanks very much in advance

CU

Manuel

Re: Problems with reading DVD using 2.6.21.5

[Robert Hancock]

Manuel Reimer wrote:
> Hello,
> 
> today I've tried to install Slackware 12.0
> 
> As the installer just "skipped" some install steps, I tried to find the 
> error.
> 
> The problem seems to be unreadable parts on the DVD:
> 
> http://pastebin.com/f381e8a88
> 
> But the DVD is OK. I've checked the MD5sum directly from disc on the 
> same system using the same DVD drive.
> 
> dmesg says:
> 
> http://pastebin.com/f63c5c389
> 
> The kernel, used on the Slackware setup disk, uses SMP, but my hardware 
> doesn't support this (get error on dmesg). May this (SMP kernel on 
> non-SMP system) cause such bugs?
> 
> Is this a known bug? How could code, which breaks DVD access, get into 
> stable 2.6.21.5?

I don't think this is a bug, the drive was told to read a sector and 
returned error SK=03, ASC=02, ASCQ=00 which is "NO SEEK COMPLETE", in 
other words it couldn't find that sector. Could be that the disc is 
marginally readable and only sometimes causes read errors.

-- 
Robert Hancock      Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@nospamshaw.ca
Home Page: http://www.roberthancock.com/

Re: Problems with reading DVD using 2.6.21.5

[Manuel Reimer]

Robert Hancock wrote:
> I don't think this is a bug, the drive was told to read a sector and 
> returned error SK=03, ASC=02, ASCQ=00 which is "NO SEEK COMPLETE", in 
> other words it couldn't find that sector. Could be that the disc is 
> marginally readable and only sometimes causes read errors.

The disc is OK. No scratches and no fingerprints on the data side. The 
"position" of this error also seems to be not static. One boot before 
the one, where I saved the "ls -l" output, the directory "kde" has been 
unreadable. I unmounted and remounted the disc and immediately after 
that, all directories have been readable, again.

I was able to read exactly this disc with exactly this drive to get the 
MD5sum of the ISO on the disc. The MD5sum was OK. Even the gpgverify, I 
did immediately after the MD5sum, was OK. No read errors or any other 
problems.

The major problem is, that this one drove me nuts while installing 
Slackware, as I always was unsure if this silly installer now installed 
anything correctly or if I had read errors, again.

Why is this error gone after unmount/mount? Why isn't the kernel able to 
"reread" (retry) a part of the DVD, without the need to remount the 
whole disc?

Thanks very much in advance

CU

Manuel

Re: Problems with reading DVD using 2.6.21.5

[Manuel Reimer]

Hello,

I tried to reproduce this bug by mounting/unmounting the drive several 
times.

It seems like this problem is caused, if the drive is at speed 0 after 
some time without access. In this situation, sometimes the drive seems 
to take a bit longer to speed up. Whenever it takes a bit longer, I get 
the error in dmesg and parts of the mounted disc are unreadable...

Any chance to get this debugged?

CU

Manuel

Re: Problems with reading DVD using 2.6.21.5

[Robert Hancock]

Manuel Reimer wrote:
> Hello,
> 
> I tried to reproduce this bug by mounting/unmounting the drive several 
> times.
> 
> It seems like this problem is caused, if the drive is at speed 0 after 
> some time without access. In this situation, sometimes the drive seems 
> to take a bit longer to speed up. Whenever it takes a bit longer, I get 
> the error in dmesg and parts of the mounted disc are unreadable...
> 
> Any chance to get this debugged?

It's not entirely clear to me whether the kernel is doing any retries or 
not. It likely should be, but somebody more familiar with the block 
layer would likely have to answer whether it will be or not..

Certainly if we are already retrying and the drive keeps saying "no seek 
complete" there's not much the kernel can do about it..

-- 
Robert Hancock      Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@nospamshaw.ca
Home Page: http://www.roberthancock.com/

Re: Problems with reading DVD using 2.6.21.5

[Manuel Reimer]

Robert Hancock wrote:
> It's not entirely clear to me whether the kernel is doing any retries or 
> not. It likely should be, but somebody more familiar with the block 
> layer would likely have to answer whether it will be or not..

Would be pretty great to get answer of someone, who knows if the kernel 
retries on errors.

As remounting fixes the problem, I think it doesn't have to do anything 
with the IDE driver, right? The device node should always be connected 
with the physical device, even if unmount. So the "problem" seems to be 
in the part, which converts "device node" to "readable filesystem" by 
mounting the device node to mount point (ISO 9660 driver?). Seems like 
this part gets restarted by remounting.

So the question is: What exactly will this "ISO 9660 driver" do, if 
there is a read error? Will it mark this "failed byte" as "unreadable" 
and will never try again, or will it try again for at least one ore more 
times?

> Certainly if we are already retrying and the drive keeps saying "no seek 
> complete" there's not much the kernel can do about it..

Thats, of course, true, but it would be pretty interesting to know, if 
the kernel does retrying...

Thanks in advance

CU

Manuel

Re: Problems with reading DVD using 2.6.21.5

[Alan Cox]

> As remounting fixes the problem, I think it doesn't have to do anything 
> with the IDE driver, right? The device node should always be connected 
> with the physical device, even if unmount. So the "problem" seems to be 

The IDE driver handles retries, although for IDE the drive itself also
handles them too by default.

> So the question is: What exactly will this "ISO 9660 driver" do, if 
> there is a read error? Will it mark this "failed byte" as "unreadable" 
> and will never try again, or will it try again for at least one ore more 
> times?

It knows nothing about retry - that happens lower down.