-This is intended for upgrading the database format used by
-Xapian. It DOES NOT upgrade the schema used by the
-public-inbox search interface (see L<public-inbox-index(1)>).
+=over
+
+=item --compact
+
+In addition to performing the copy operation, run L<xapian-compact(1)>
+on each Xapian shard after copying but before finalizing it.
+Compared to the cost of copying a Xapian database, compacting a
+Xapian database takes only around 5% of the time required to copy.
+
+Compared to L<public-inbox-compact(1)>, use of this option is
+preferable for gigantic inboxes where the coarse-grained lock
+currently required for L<public-inbox-compact(1)> can cause
+the compaction to take hours at-a-time.
+
+=item --reshard=N / -R N
+
+Reshard the Xapian database on a L<v2|public-inbox-v2-format(5)>
+inbox to C<N> shards . Since L<xapian-compact(1)> is not suitable
+for merging, users can rely on this switch to reshard the
+existing Xapian database(s) to any positive value of C<N>.
+
+This is useful in case the Xapian DB was created with too few or
+too many shards given the capabilities of the current hardware.
+
+=item --blocksize / --no-full / --fuller
+
+These options are passed directly to L<xapian-compact(1)> when
+used with C<--compact>.
+
+=back