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. James -- To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
