I have a master-slave configuration. When I install a fresh application I manually run the indexing on the master, and after a short time I get two indexes (the master and the copy) that are about 200MB in size.
After one day, with hardly no application usage, the index has grown to 12 GB, filling up the hard disk.
This happens constantly.

What could be the problem?

hibernate.search.default.directory_provider = filesystem-master
hibernate.search.default.indexBase = /mpath/lucene/indexes
hibernate.search.default.sourceBase = /mpath/lucene/copy

I'm using Hibernate Search 3.4.2.Final on Hibernate Core 3.6.0 and Lucene Core 3.1.0

hi, you're using old version of libraries. In particular in that version of Lucene there where some file handles leaks under specific stress conditions, I think that might be preventing proper cleanup when segments should be deleted.