+manifest=$(mktemp manifestXXXXXX)
+
+find mail -type f ! -path 'mail/.notmuch/*' | sed -n '1~4 p' > $manifest
+# arithmetic context is to eat extra whitespace on e.g. some BSDs
+count=$((`wc -l < $manifest`))
+
+xargs tar uf backup.tar < $manifest
+
+perl -nle 'rename $_, "$_.renamed"' $manifest
+
+time_run "reindex ($count mv)" "notmuch reindex '*'"
+
+perl -nle 'rename "$_.renamed", $_' $manifest
+
+time_run "reindex ($count mv back)" "notmuch reindex '*'"
+
+perl -nle 'unlink $_; unlink $_.copy' $manifest
+
+time_run "reindex ($count rm)" "notmuch reindex '*'"
+
+tar xf backup.tar
+
+time_run "reindex ($count restore)" "notmuch reindex '*'"
+
+perl -nle 'link $_, "$_.copy"' $manifest
+
+time_run "reindex ($count cp)" "notmuch reindex '*'"
+