What version are you on ?
Check the result of you major compaction by looking for log lines such as "Compacted to…" They will say how much smaller the new file is.
After a major compaction there should be a single SSTable, the ks-cf-he-1234 part with multiple components such as -Data.db. How many files do you have for the CF on disk ? Are you also using secondary indexes ?
Reducing gc and forcing a major compaction should result in all purgable space being removed. There are times when minor compaction cannot purge expired tombstones because the rows are spread out over multiple size tiers.
Expanding from 5GB to 70Gb is out of the normal expections I would say. You may want to check it the db contains what you expect it to.
I'm using size-tiered compaction.
2012/9/3 Derek Williams <firstname.lastname@example.org>
Does anyone have any suggestions on how can I analyze my problem? Or maybe I'm doing something wrong and there is another way to force gc on an existing column family.
Are you using leveled compaction? I haven't looked into it too much, but I think forcing a major compaction when using leveled strategy doesn't have the same effect as with size tiered.