The blackhole storage engine

MySQL supports multiple storage engines, such as MyISAM and InnoDB. One such storage engine is the BLACKHOLE storage engine. Everything you write to a table in this storage engine goes to /dev/null, in other words, it doesn't go anywhere.

So, what is this useful for?

So, while the actual table content isn't stored, the binary log does still exist (the binary log stores a list of modifications to a table such that if the db crashes you don't lose anything). You can therefore use this to replicate the blackhole table to another table somewhere else that uses a more normal storage engine (recall that when you replicate a table, you don't have to use the same storage engine on each side). This allows you to insert into a table asynchronously according to your replication settings.