From the design doc, mob files are not involved in the normal HBase compaction process. This means deleted mobs would still take up space and that we never really merge mob files that accrue over time. Currently, MOBs depend on two external tools:
1) A TTL cleaner that removes mobs that have passed their TTL or exceeded minVersions.
2) A 'sweep tool' cleaner that remove mobs that have had their references deleted and merges small files into larger ones.
Today the tools are triggered by admins. The longer term goal would be to integrate them into hbase such that by default mobs are cleaned. The tools will be preserved however so that advanced admins can disable automatic cleanups and manually trigger these compaction like operaitons. #1 would likely be a chore in the master while #2 requires some design work to integrate into hbase.