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

include/sys/fm/fs/zfs.h

@@ -23,6 +23,10 @@
  * Use is subject to license terms.
  */
 
+/*
+ *  Copyright (c) 2020 by Delphix. All rights reserved.
+ */
+
 #ifndef	_SYS_FM_FS_ZFS_H
 #define	_SYS_FM_FS_ZFS_H
 
@@ -88,6 +92,7 @@ extern "C" {
 #define	FM_EREPORT_PAYLOAD_ZFS_ZIO_SIZE		"zio_size"
 #define	FM_EREPORT_PAYLOAD_ZFS_ZIO_FLAGS	"zio_flags"
 #define	FM_EREPORT_PAYLOAD_ZFS_ZIO_STAGE	"zio_stage"
+#define	FM_EREPORT_PAYLOAD_ZFS_ZIO_PRIORITY	"zio_priority"
 #define	FM_EREPORT_PAYLOAD_ZFS_ZIO_PIPELINE	"zio_pipeline"
 #define	FM_EREPORT_PAYLOAD_ZFS_ZIO_DELAY	"zio_delay"
 #define	FM_EREPORT_PAYLOAD_ZFS_ZIO_TIMESTAMP	"zio_timestamp"

include/sys/fm/util.h

@@ -104,6 +104,9 @@ extern int zfs_zevent_seek(zfs_zevent_t *, uint64_t);
 extern void zfs_zevent_init(zfs_zevent_t **);
 extern void zfs_zevent_destroy(zfs_zevent_t *);
 
+extern void zfs_zevent_track_duplicate(void);
+extern void zfs_ereport_init(void);
+extern void zfs_ereport_fini(void);
 #else
 
 static inline void fm_init(void) { }