I found that mongod writes
[snapshotthread] cpu: elapsed:4000 writelock: 0%
So it one day log now about 2Mb. What does it mean and how to disable this?
The log entries you're seeing are generated when you enable cpu logging in MongoDB, i.e. starting it with the --cpu flag or cpu = true in the config file. Disabling that option will prevent it from adding those entries to the log file.
You can read about that here: http://docs.mongodb.org/manual/reference/configuration-options/#cpu