OpenZFS 8961 - SPA load/import should tell us why it failed

Problem
=======

When we fail to open or import a storage pool, we typically don't
get any additional diagnostic information, just "no pool found" or
"can not import".

While there may be no additional user-consumable information, we should
at least make this situation easier to debug/diagnose for developers
and support.  For example, we could start by using `zfs_dbgmsg()`
to log each thing that we try when importing, and which things
failed. E.g. "tried uberblock of txg X from label Y of device Z". Also,
we could log each of the stages that we go through in `spa_load_impl()`.

Solution
========

Following the cleanup to `spa_load_impl()`, debug messages have been
added to every point of failure in that function. Additionally,
debug messages have been added to strategic places, such as
`vdev_disk_open()`.

Authored by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Andrew Stormont <andyjstormont@gmail.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Ported-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Tim Chase <tim@chase2k.com>

OpenZFS-issue: https://illumos.org/issues/8961
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/418079e0
Closes #7459

1 files changed, 9 insertions, 2 deletions

diff --git a/module/zfs/vdev_label.c b/module/zfs/vdev_label.c
index b91e332de39f..ad334fe8c2c6 100644
--- a/module/zfs/vdev_label.c
+++ b/module/zfs/vdev_label.c
@@ -21,7 +21,7 @@
 
 /*
  * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
- * Copyright (c) 2012, 2015 by Delphix. All rights reserved.
+ * Copyright (c) 2012, 2016 by Delphix. All rights reserved.
  */
 
 /*
@@ -1204,8 +1204,15 @@ vdev_uberblock_load(vdev_t *rvd, uberblock_t *ub, nvlist_t **config)
 	 * Search all labels on this vdev to find the configuration that
 	 * matches the txg for our uberblock.
 	 */
-	if (cb.ubl_vd != NULL)
+	if (cb.ubl_vd != NULL) {
+		vdev_dbgmsg(cb.ubl_vd, "best uberblock found for spa %s. "
+		    "txg %llu", spa->spa_name, (u_longlong_t)ub->ub_txg);
+
 		*config = vdev_label_read_config(cb.ubl_vd, ub->ub_txg);
+		if (*config == NULL) {
+			vdev_dbgmsg(cb.ubl_vd, "failed to read label config");
+		}
+	}
 	spa_config_exit(spa, SCL_ALL, FTAG);
 }