zpool import progress kstat

When an import requires a long MMP activity check, or when the user
requests pool recovery, the import make take a long time.  The user may
not know why, or be able to tell whether the import is progressing or is
hung.

Add a kstat which lists all imports currently being processed by the
kernel (currently only one at a time is possible, but the kstat allows
for more than one).  The kstat is /proc/spl/kstat/zfs/import_progress.

The kstat contents are as follows:
pool_guid         load_state multihost_secs  max_txg pool_name
16667015954387398 3          15              0       tank3

load_state: the value of spa_load_state
multihost_secs:  seconds until the end of the multihost activity
                 check; if over, or none required, this is 0
max_txg: current spa_load_max_txg, if rewind is occurring

This could be used by outside tools, such as a pacemaker resource agent,
to report import progress, or as a part of manual troubleshooting.  The
zpool import subcommand could also be modified to report this
information.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #8696

include/sys/spa.h

@@ -965,6 +965,14 @@ extern void spa_iostats_trim_add(spa_t *spa, trim_type_t type,
     uint64_t extents_written, uint64_t bytes_written,
     uint64_t extents_skipped, uint64_t bytes_skipped,
     uint64_t extents_failed, uint64_t bytes_failed);
+extern void spa_import_progress_add(spa_t *spa);
+extern void spa_import_progress_remove(uint64_t spa_guid);
+extern int spa_import_progress_set_mmp_check(uint64_t pool_guid,
+    uint64_t mmp_sec_remaining);
+extern int spa_import_progress_set_max_txg(uint64_t pool_guid,
+    uint64_t max_txg);
+extern int spa_import_progress_set_state(uint64_t pool_guid,
+    spa_load_state_t spa_load_state);
 
 /* Pool configuration locks */
 extern int spa_config_tryenter(spa_t *spa, int locks, void *tag, krw_t rw);