Avoid posting duplicate zpool events

Duplicate io and checksum ereport events can misrepresent that 
things are worse than they seem. Ideally the zpool events and the 
corresponding vdev stat error counts in a zpool status should be 
for unique errors -- not the same error being counted over and over. 
This can be demonstrated in a simple example. With a single bad 
block in a datafile and just 5 reads of the file we end up with a 
degraded vdev, even though there is only one unique error in the pool.

The proposed solution to the above issue, is to eliminate duplicates 
when posting events and when updating vdev error stats. We now save 
recent error events of interest when posting events so that we can 
easily check for duplicates when posting an error. 

Reviewed by: Brad Lewis <brad.lewis@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Don Brady <don.brady@delphix.com>
Closes #10861

module/zfs/fm.c

@@ -104,13 +104,15 @@ struct erpt_kstat {
 	kstat_named_t	erpt_set_failed;	/* num erpt set failures */
 	kstat_named_t	fmri_set_failed;	/* num fmri set failures */
 	kstat_named_t	payload_set_failed;	/* num payload set failures */
+	kstat_named_t	erpt_duplicates;	/* num duplicate erpts */
 };
 
 static struct erpt_kstat erpt_kstat_data = {
 	{ "erpt-dropped", KSTAT_DATA_UINT64 },
 	{ "erpt-set-failed", KSTAT_DATA_UINT64 },
 	{ "fmri-set-failed", KSTAT_DATA_UINT64 },
-	{ "payload-set-failed", KSTAT_DATA_UINT64 }
+	{ "payload-set-failed", KSTAT_DATA_UINT64 },
+	{ "erpt-duplicates", KSTAT_DATA_UINT64 }
 };
 
 kstat_t *fm_ksp;
@@ -568,6 +570,12 @@ out:
 	return (error);
 }
 
+void
+zfs_zevent_track_duplicate(void)
+{
+	atomic_inc_64(&erpt_kstat_data.erpt_duplicates.value.ui64);
+}
+
 static int
 zfs_zevent_minor_to_state(minor_t minor, zfs_zevent_t **ze)
 {
@@ -1633,6 +1641,8 @@ fm_init(void)
 	list_create(&zevent_list, sizeof (zevent_t),
 	    offsetof(zevent_t, ev_node));
 	cv_init(&zevent_cv, NULL, CV_DEFAULT, NULL);
+
+	zfs_ereport_init();
 }
 
 void
@@ -1640,6 +1650,8 @@ fm_fini(void)
 {
 	int count;
 
+	zfs_ereport_fini();
+
 	zfs_zevent_drain_all(&count);
 
 	mutex_enter(&zevent_lock);