On Wed, 2007-01-31 at 10:13 -0500, Mark Lord wrote:

James Bottomley wrote:

quoted

For the MD case, this is what REQ_FAILFAST is for.

I cannot find where SCSI honours that flag.  James?

Er, it's in scsi_error.c:scsi_decide_disposition():

      maybe_retry:

	/* we requeue for retry because the error was retryable, and
	 * the request was not marked fast fail.  Note that above,
	 * even if the request is marked fast fail, we still requeue
	 * for queue congestion conditions (QUEUE_FULL or BUSY) */
	if ((++scmd->retries) <= scmd->allowed
	    && !blk_noretry_request(scmd->request)) {
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
		return NEEDS_RETRY;
	} else {
		/*
		 * no more retries - report this one back to upper level.
		 */
		return SUCCESS;
	}

And for that matter, even when I patch SCSI so that it *does* honour it,
I don't actually see the flag making it into the SCSI layer from above.

And I don't see where/how the block layer takes care when considering
merge FAILFAST/READA requests with non FAILFAST/READA requests.
To me, it looks perfectly happy to add non-FAILFAST/READA bios
to a FAILFAST request, risking data loss if a lower-layer decides
to honour the FAILFAST/READA flags.

So it's a pretty Good Thing(tm) that SCSI doesn't currently honour it. ;)

James