On Thu, 19 Mar 2009 23:32:55 -0400 Mark Lord <liml@xxxxxx> 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@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;
> + }
> 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?
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