On Thu, 2009-03-19 at 23:32 -0400, Mark Lord wrote: > Norman Diamond wrote: > > For months I was wondering how a disk could do this: > > dd if=/dev/hda of=/dev/null bs=512 skip=551540 count=4 # succeeds > > dd if=/dev/hda of=/dev/null bs=512 skip=551544 count=4 # succeeds > > dd if=/dev/hda of=/dev/null bs=512 skip=551540 count=8 # fails This basically means the drive doesn't report where in the requested transfer the error occurred. If we have that information, we'd return all sectors up to that LBA as OK and all at or beyond as -EIO, so the readahead wouldn't matter. > > It turns out the disk isn't doing that. Linux is. The old IDE drivers did > > it, but with LIBATA the same thing happens to /dev/sda. In later examples > > also, the same happens to /dev/sda as /dev/hda. > .. > > You can blame me for the IDE driver not doing that properly. > But for libata, it's the SCSI layer. > > I've been patching this for years for my clients, > and will be updating the patch soon-ish and trying > again to get it into upstream kernels. > > Here's the (now ancient) 2.6.20 version for SLES10: > > * * * > > Allow SCSI to continue with the remaining blocks of a request > after encountering a media error. Otherwise, it may just fail > the entire request, even though some blocks were fine and needed > by a completely different process than the one that wanted the bad block(s). > > Signed-off-by: Mark Lord <mlord@xxxxxxxxx> > > --- linux-2.6.16.60-0.6/drivers/scsi/scsi_lib.c 2008-03-10 13:46:03.000000000 -0400 > +++ linux/drivers/scsi/scsi_lib.c 2008-03-21 11:54:09.000000000 -0400 > @@ -888,6 +888,12 @@ > */ > if (sense_valid && !sense_deferred) { > switch (sshdr.sense_key) { > + case MEDIUM_ERROR: > + /* Bad sector. Fail it, and then continue the rest of the request. */ > + if (scsi_end_request(cmd, 0, cmd->device->sector_size, 1) == NULL) { > + cmd->retries = 0; // go around again.. > + return; > + } But we've been over this. You can't apply something like this because it ignores retries and chunks up the request a sector at a time. For the enterprise that can increase failure time from a few seconds to hours for 512k transfers. Using the disk supplied data about where the error occurred (provided the disk returns it) eliminates all the readahead problems like the one above. Perhaps just turning of readahead for disks that don't supply error location information would be a reasonable workaround? 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