Update Tools.markdown

doc/Tools.markdown

@@ -62,6 +62,9 @@ vtworker is meant to host long-running processes. It supports a plugin infrastru
 
 It is very easy to add other checker processes for in-tablet integrity checks (verifying foreign key-like relationships), and cross shard data integrity (for instance, if a keyspace contains an index table referencing data in another keyspace).
 
+### vtprimecache
+vtprimecache is a mysql cache primer for faster replication. If the single MySQL replication thread is falling behind, vtprimecache activates and starts reading the available relay logs. If then uses a few threads / connections to MySQL to execute modified statements and prime the MySQL buffer cache. The idea is for instance if an 'update table X where id=2' statement is going to be executed by the replication thread 2 or 3 seconds from now, might as well execute a concurrent 'select from table X wherre id=2' now and prime MySQL cache. In practice, this shows a speed improvement in replication speed by 30 to 40 percents.
+
 ### Other support tools
 * *mysqlctl*: manage MySQL instances.
 * *zkctl*: manage ZooKeeper instances.