New INFO field aof_delayed_fsync introduced.

This new field counts all the times Redis is configured with AOF enabled and
fsync policy 'everysec', but the previous fsync performed by the
background thread was not able to complete within two seconds, forcing
Redis to perform a write against the AOF file while the fsync is still
in progress (likely a blocking operation).

src/aof.c

@@ -108,6 +108,7 @@ void flushAppendOnlyFile(int force) {
             }
             /* Otherwise fall trough, and go write since we can't wait
              * over two seconds. */
+            server.aof_delayed_fsync++;
             redisLog(REDIS_NOTICE,"Asynchronous AOF fsync is taking too long (disk is busy?). Writing the AOF buffer without waiting for fsync to complete, this may slow down Redis.");
         }
     }