Re: Overagressive failing of disk reads, both LIBATA and IDE

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 19 Mar 2009 23:32:55 -0400 Mark Lord <liml@rtr.ca> 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
> > 
> > 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@pobox.com>
> 
> --- 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;
> +		}
>  		case UNIT_ATTENTION:
>  			if (cmd->device->removable) {
>  				/* Detected disc change.  Set a bit

Once upon a time the VFS would fall back to single page reads when a large
readahead request failed.  That's probably still the case.

It was more by accident than by design, but it had (has) the desired effect?