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

man/man5/zfs-module-parameters.5

@@ -1,6 +1,6 @@
 '\" te
 .\" Copyright (c) 2013 by Turbo Fredriksson <turbo@bayour.com>. All rights reserved.
-.\" Copyright (c) 2019 by Delphix. All rights reserved.
+.\" Copyright (c) 2019, 2020 by Delphix. All rights reserved.
 .\" Copyright (c) 2019 Datto Inc.
 .\" The contents of this file are subject to the terms of the Common Development
 .\" and Distribution License (the "License").  You may not use this file except
@@ -3656,6 +3656,27 @@ Default value: \fB0\fR.
 
 .sp
 .ne 2
+.na
+\fBzfs_zevent_retain_max\fR (int)
+.ad
+.RS 12n
+Maximum recent zevent records to retain for duplicate checking.  Setting
+this value to zero disables duplicate detection.
+.sp
+Default value: \fB2000\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_zevent_retain_expire_secs\fR (int)
+.ad
+.RS 12n
+Lifespan for a recent ereport that was retained for duplicate checking.
+.sp
+Default value: \fB900\fR.
+.RE
+
 .na
 \fBzfs_zil_clean_taskq_maxalloc\fR (int)
 .ad