+and Xapian. This is only effective with Xapian 1.4+. This is
+primarily intended for systems with low RAM and the small
+(default) C<--batch-size=1m>. Users of large C<--batch-size>
+may even find disabling L<fdatasync(2)> causes too much dirty
+data to accumulate, resulting on latency spikes from writeback.
+
+Available in public-inbox 1.6.0+.
+
+=item --dangerous
+
+Speed up initial index by using in-place updates and denying support for
+concurrent readers. This is only effective with Xapian 1.4+.
+
+Available in public-inbox 1.8.0+
+
+=item --sequential-shard
+
+Sets or overrides L</publicinbox.indexSequentialShard> on a
+per-invocation basis. See L</publicinbox.indexSequentialShard>
+below.
+
+Available in public-inbox 1.6.0+.
+
+=item --skip-docdata
+
+Stop storing document data in Xapian on an existing inbox.
+
+See L<public-inbox-init(1)/--skip-docdata> for description and caveats.
+
+Available in public-inbox 1.6.0+.
+
+=item -E EXTINDEX
+
+=item --update-extindex=EXTINDEX
+
+Update the given external index (L<public-inbox-extindex-format(5)>.
+Either the configured section name (e.g. C<all>) or a directory name
+may be specified.
+
+Defaults to C<all> if C<[extindex "all"]> is configured,
+otherwise no external indices are updated.
+
+May be specified multiple times in rare cases where multiple
+external indices are configured.