List log_spacemap feature in zpool-features.5 manual

Update zpool-features.5 manpage to describe the log_spacemap feature.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Pavel Zakharov <pavel.zakharov@delphix.com>
Signed-off-by: Serapheim Dimitropoulos <serapheim@delphix.com>
Closes #9096

diff --git a/man/man5/zpool-features.5 b/man/man5/zpool-features.5
index 00bad9f74ed06eff26fa17554dd1c091ce1087ae..6c03f9049066d9b59eca7fc523dc69b958583801 100644 (file)
--- a/man/man5/zpool-features.5
+++ b/man/man5/zpool-features.5
@@ -567,6 +567,28 @@ The feature is activated when a clone is created and remains active until all
 clones have been destroyed.
 .RE
 
+.sp
+.ne 2
+.na
+\fBlog_spacemap\fR
+.ad
+.RS 4n
+.TS
+l l .
+GUID   com.delphix:log_spacemap
+READ\-ONLY COMPATIBLE  yes
+DEPENDENCIES   com.delphix:spacemap_v2
+.TE
+
+This feature improves performance for heavily-fragmented pools,
+especially when workloads are heavy in random-writes. It does so by
+logging all the metaslab changes on a single spacemap every TXG
+instead of scattering multiple writes to all the metaslab spacemaps.
+
+This feature becomes \fBactive\fR as soon as it is enabled and will never
+return to being \fBenabled\fR.
+.RE
+
 .sp
 .ne 2
 .na