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 <[email protected]>
Reviewed by: George Wilson <[email protected]>
Reviewed by: Matthew Ahrens <[email protected]>
Reviewed by: Andrew Stormont <[email protected]>
Approved by: Dan McDonald <[email protected]>
Ported-by: Tim Chase <[email protected]>
Signed-off-by: Tim Chase <[email protected]>

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

1 files changed, 28 insertions, 0 deletions

diff --git a/module/zfs/spa_misc.c b/module/zfs/spa_misc.c
index 76436ba5c..6f4db76c8 100644
--- a/module/zfs/spa_misc.c
+++ b/module/zfs/spa_misc.c
@@ -373,6 +373,34 @@ int spa_asize_inflation = 24;
 int spa_slop_shift = 5;
 uint64_t spa_min_slop = 128 * 1024 * 1024;
 
+/*PRINTFLIKE2*/
+void
+spa_load_failed(spa_t *spa, const char *fmt, ...)
+{
+	va_list adx;
+	char buf[256];
+
+	va_start(adx, fmt);
+	(void) vsnprintf(buf, sizeof (buf), fmt, adx);
+	va_end(adx);
+
+	zfs_dbgmsg("spa_load(%s): FAILED: %s", spa->spa_name, buf);
+}
+
+/*PRINTFLIKE2*/
+void
+spa_load_note(spa_t *spa, const char *fmt, ...)
+{
+	va_list adx;
+	char buf[256];
+
+	va_start(adx, fmt);
+	(void) vsnprintf(buf, sizeof (buf), fmt, adx);
+	va_end(adx);
+
+	zfs_dbgmsg("spa_load(%s): %s", spa->spa_name, buf);
+}
+
 /*
  * ==========================================================================
  * SPA config locking