hash: drop bees_unreadahead

Forcing the entire hash table into immediate writeback causes crippling
write latencies at shutdown.  Even discarding pages as they are read in
at startup can trigger a writeback latency spike if the pages are dirty
at read time.

Better to let the VM subsystem handle this on its own.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

src/bees-hash.cc

@@ -206,8 +206,11 @@ BeesHashTable::writeback_loop()
 	}
 	catch_all([&]() {
 		// trigger writeback on our way out
+#if 0
+		// seems to trigger huge latency spikes
 		BEESTOOLONG("unreadahead hash table size " << pretty(m_size));
 		bees_unreadahead(m_fd, 0, m_size);
+#endif
 	});
 	BEESLOGDEBUG("Exited hash table writeback_loop");
 }