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SubjectRe: Overagressive failing of disk reads, both LIBATA and IDE
On Sat, 2009-03-21 at 15:10 +0000, Alan Cox wrote:
> > Using the disk supplied data about where the error occurred (provided
> > the disk returns it) eliminates all the readahead problems like the one
> ATA disks do provide sector information generally, as do CD-ROMs, in fact
> we actually decode it for error reporting so probably all the bits are
> there to improve any reporting for most read side cases.
> >From some of the traces I have been debugging I am not convinced the scsi
> mid layer does the right thing any more. It uses to be handling CD-R
> media (where the end of media is not well defined) but nowdays seems to
> be reporting errors even when told that the I/O partly succeeded. I need
> to debug that case further however but as I don't have one of the problem
> drives its a bit tricky.

OK, so in modern kernels, this is done in the ULD ... specifically for
disks in sd.c:sd_done and for CD/DVD in sr.c:sr_done().

The sr.c one looks crufty in that I don't think it handles descriptor
sense at all, so perhaps it should be updated to match the sd one?

> > above. Perhaps just turning of readahead for disks that don't supply
> > error location information would be a reasonable workaround?
> Not really. From a performance perspective Mark's patch is vastly
> superior because it punishes the incredibly rare error case not the
> routine working performance. Avoiding the need to do either would be even
> better - as would fixing the block layer not to mess up retries in this
> situation.
> For low speed devices like MMC cards and flash it might make sense not to
> merge unrelated requests however so that only the relevant one fails.

See other email for suggestion how to do this at the block level.


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