Slave delay can be a nightmare. I battle it every day and know plenty of people who curse the serialization problem of replication. For those who are not familiar with it, replication on MySQL slaves runs commands in series – one by one, while the master may run them in parallel. This fact usually causes bottlenecks. Consider these 2 examples:

• Between 1 and 100 UPDATE queries are constantly running on the master in parallel. If the slave IO is only fast enough to handle 50 of them without lagging, as soon as 51 start running, the slaves starts to lag.
• A more common problem is when one query takes an hour to run (let's say, it's an UPDATE with a big WHERE clause that doesn't use an index). In this case, the query runs on the master for an

MySQL Slave Delay implements an oft-requested replication feature: the ability to make a slave lag its master, a.k.a scheduling binlog events for some time in the future. This will probably be built into MySQL replication someday, but in the meantime, you can use this tool. A delayed slave is great if disaster strikes and propagates through replication, because it lets you stop the delayed slave before it gets corrupted, restart the slave until just before the offending statement, and recover quickly. This is a lot faster than restoring last night's full backup and replaying a day's worth of binary logs on the master.