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Re: the media check problem



On Sat, 13 Nov 2004, Alan Cox wrote:

> > 1) It's the read ahead that causes the problem. If one use
> >    hdparm and set the read ahead to 0 on the device, then I have
> >    no problem to verify a cd using dd.
> 
> Correct

So, why not set read ahead to 0 during the install process (or at least 
during the media check).

It's also strange that the media check works for some people.

I don't see anything here that depends on the hardware except perhaps
hardware that does not support dma at all (blacklisted), or that does not
use ide-cd. Maybe it's the combination of slow CPU and fast cd-rom that
makes it perform more read ahead and increase the chance of generating an
error (yet another guess :-).

> The "end" of a CD-R is undefined for about 150K. ide-scsi correctly 
> retries errors at the disk end and reduces the disk size based on the
> error position. ide-cd doesn't.

If the layer above can tell ide-cd where the end is then it will never
need to seek past that. For an iso fs the size is known and I guessed in
the previous mail that this size was passed in to ide-cd when you mount
it.

A retry to find the real end of course would also solve it.

I don't know that much about cdroms so I can't say I know anything about
the "end is undefined for 150K" part. I played with different padding
sizes, but that didn't alter the end position. The full track I could read
back in was of constant size no matter what padding I tried.

Well, I feel a lot better now that I know what the problem is. I hate 
problems where I don't know the technical reason behind it. Thanks.

-- 
/Dennis Björklund


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