Illumos #4101 finer-grained control of metaslab_debug

Today the metaslab_debug logic performs two tasks:

- load all metaslabs on import/open
- don't unload metaslabs at the end of spa_sync

This change provides knobs for each of these independently.

References:
  https://illumos.org/issues/4101
  https://github.com/illumos/illumos-gate/commit/0713e23

Notes:

1) This is a small piece of the metaslab improvement patch from
Illumos. It was worth bringing over before the rest, since it's
low risk and it can be useful on fragmented pools (e.g. Lustre
MDTs). metaslab_debug_unload would give the performance benefit
of the old metaslab_debug option without causing unwanted delay
during pool import.

Ported-by: Ned Bass <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #2227

1 files changed, 13 insertions, 2 deletions

diff --git a/man/man5/zfs-module-parameters.5 b/man/man5/zfs-module-parameters.5
index e0d44d22c..815c856c0 100644
--- a/man/man5/zfs-module-parameters.5
+++ b/man/man5/zfs-module-parameters.5
@@ -137,10 +137,21 @@ Default value: \fB8,388,608\fR.
 .sp
 .ne 2
 .na
-\fBmetaslab_debug\fR (int)
+\fBmetaslab_debug_load\fR (int)
 .ad
 .RS 12n
-Keep space maps in core to verify frees
+Load all metaslabs during pool import.
+.sp
+Use \fB1\fR for yes and \fB0\fR for no (default).
+.RE
+
+.sp
+.ne 2
+.na
+\fBmetaslab_debug_unload\fR (int)
+.ad
+.RS 12n
+Prevent metaslabs from being unloaded.
 .sp
 Use \fB1\fR for yes and \fB0\fR for no (default).
 .RE