cam/da: Document what an 'invalid pack' means.

An 'invalid pack' in the da driver means that the media for the drive is
temproarily missing, pending an action by a third party (eg robot arm or
human inserting a disk pack, etc) or has changed since we opened. We
invalidate a pack when the ASC/ASCQ tells us the media is missing (ASC
0x3a MEDIA MISSING), for 0x28/0 (NOT READY TO READY CHANGE, MEDIUM MAY
HAVE CHANGED) and for that brief period of time after a peripheral has
been invalidated before the disk is destroyed. When the pack is invalid,
we can't do READ/WRITE operations to the media, so we fail all
outstanding I/O and any new I/O that comes in when this happens.  This
is cleared in daopen when we've repobed the device.

Sponsored by:		Netflix
Reviewed by:		mav
Differential Revision:	https://reviews.freebsd.org/D48687

1 files changed, 13 insertions, 1 deletions

diff --git a/sys/cam/scsi/scsi_da.c b/sys/cam/scsi/scsi_da.c
index e8e13613413a..f2f3951ebe80 100644
--- a/sys/cam/scsi/scsi_da.c
+++ b/sys/cam/scsi/scsi_da.c
@@ -1899,7 +1899,15 @@ dastrategy(struct bio *bp)
 	cam_periph_lock(periph);
 
 	/*
-	 * If the device has been made invalid, error out
+	 * If the pack has been invalidated, fail all I/O. The medium is not
+	 * suitable for normal I/O, because one or more is ture:
+	 *	- the medium is missing
+	 *	- its size is unknown
+	 *	- it differs from the medium present at daopen
+	 *	- we're tearing the cam periph device down
+	 * Since we have the cam periph lock, we don't need to check it for
+	 * the last condition since PACK_INVALID is set when we invalidate
+	 * the device.
 	 */
 	if ((softc->flags & DA_FLAG_PACK_INVALID)) {
 		cam_periph_unlock(periph);
@@ -1946,6 +1954,10 @@ dadump(void *arg, void *virtual, off_t offset, size_t length)
 	softc = (struct da_softc *)periph->softc;
 	secsize = softc->params.secsize;
 
+	/*
+	 * Can't dump to a disk that's not there or changed, for whatever
+	 * reason.
+	 */
 	if ((softc->flags & DA_FLAG_PACK_INVALID) != 0)
 		return (ENXIO);