Sweeping

Sweeping is NOT just organized garbage collection. What sweeping seeks to do is to move the Oldest Interesting Transaction up, and reduce the size of transaction masks. It does so by changing rolled back transactions to committed transactions.

"What!!!", you say. "The woman is nuts."

But that's what a sweep does. It removes all the changes made by a rolled back transaction then changes it state to committed. (Remember we agreed earlier that a read-only transaction that rolled back could be considered committed for all the harm it did. Remove the damage, and its safe to consider the transaction committed.)

At the same time, sweep garbage collects like any other transaction.

A new thread is started and sweeps the database while everybody else goes about life as normal. Well, more or less normal, where the less is the amount of CPU and I/O bandwidth used by the sweep.

Taken from IBPhoenix