Re: Silent errors when reading CDs
From: Yar Tikhiy (yar_at_comp.chem.msu.su)
Date: 07/14/04
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Date: Wed, 14 Jul 2004 03:31:21 +0400 To: Jean-Sebastien Roy <js@jeannot.org>
On Sat, Jul 10, 2004 at 12:07:40AM +0200, Jean-Sebastien Roy wrote:
>
> I'm currently using FreeBSD 4.10 on an HP D530 SFF.
> The system is perfectly stable except for the following problem
> I'm unable to understand :
>
> When I mount a cdrom (mount /cdrom), then calculate the MD5 hash
> of a big file on a CD (md5 /cdrom/bigfile), the results are often random:
> unmounting, mounting again and calculating again the MD5 often result in
> a different value. What disturb me the most is that absolutely no errors
> are reported in any log (no read errors for example).
>
> I thought the CDROM reader, a LITE-ON LTR-48327S PQS3, was the culprit,
> so I replaced it with a PLEXTOR DVDR PX-712A and got the exact same
> results (i.e. random MD5 values). I checked the RAM using memtest and
> got no errors. The problem does not occur for files on the harddisk.
Once I had an old noname PC (iP200 in an i430VX motherboard), and
I installed a DVD+RW drive into it. Data read from a CD or DVD was
damaged with high probability. With hw.ata.atapi_dma set to zero,
the probability of data corruption was lower, but still noticable.
That's while there were no corruption on burning in DMA or PIO mode.
I got used to always checksumming data read or written; it was a
real pain in the back. Finally, I dismantled the PC, but kept the
DVD-RW drive for a new PC; thus I got rid of the problem once and
for all. My conclusion was: If your hardware sucks, you may choose
to work around the bugs, but it's usually easier to dump the POS
and find a replacement. Of course, you may need to do some testing
before you know which part sucks.
> hw.ata.atapi_dma is set since both drives support it and it seems to be
> required for proper CD/DVD burning. The CD drive is the master on its
> own ATA bus.
DMA mode on an ATAPI device might be the root of the evil. Alas,
a lot of ATA controllers have bugs there. And you'll need DMA mode
only for high-speed burning, when PIO mode cannot provide enough
throughput--it's not required in general. E.g., I had DVD+Rs burned
OK at 2.4x in PIO mode on the P200. Therefore you can at least see
if disabling hw.ata.atapi_dma is a workaround for your problem.
> While the problem occurs on multiple CDs (mostly RW), to my surprise, I
> was not able to reproduce the problem by reading big files on DVDs.
Are there CDs in your collection that produce no errors?
> Could someone provide me a hint on what to check next or how to fix this
> problem ? Can errors on CDs generate such a behavior ?
Even a badly scratched CD won't produce damaged data unless read
in a brain-dead drive. If your drives work OK with a different ATA
controller, then it's probably your present ATA controller to blame.
-- Yar _______________________________________________ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "freebsd-hackers-unsubscribe@freebsd.org"
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