Make zfs_special_class_metadata_reserve_pct into a parameter

Exported and documented a new module parameter.
Reviewed-by: Richard Laager <rlaager@wiktel.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: DHE <git@dehacked.net>
Closes #8706 

man/man5/zfs-module-parameters.5

@@ -2399,6 +2399,20 @@ vdev while discarding the checkpoint.
 Default value: \fB16,777,216\fR.
 .RE
 
+.sp
+.ne 2
+.na
+\fBzfs_special_class_metadata_reserve_pct\fR (int)
+.ad
+.RS 12n
+Only allow small data blocks to be allocated on the special and dedup vdev
+types when the available free space percentage on these vdevs exceeds this
+value. This ensures reserved space is available for pool meta data as the
+special vdevs approach capacity.
+.sp
+Default value: \fB25\fR.
+.RE
+
 .sp
 .ne 2
 .na

module/zfs/spa_misc.c

@@ -2689,5 +2689,10 @@ MODULE_PARM_DESC(zfs_ddt_data_is_special,
 module_param(zfs_user_indirect_is_special, int, 0644);
 MODULE_PARM_DESC(zfs_user_indirect_is_special,
 	"Place user data indirect blocks into the special class");
+
+module_param(zfs_special_class_metadata_reserve_pct, int, 0644);
+MODULE_PARM_DESC(zfs_special_class_metadata_reserve_pct,
+	"Small file blocks in special vdevs depends on this much "
+	"free space available");
 /* END CSTYLED */
 #endif