Sorry for the wide distribution.
Our cassandra is running on 1.0.10. Recently, we are facing a weird situation. We have a column family containing wide rows (each row might have a few million of columns). We delete the columns on a daily basis and we also run major compaction on it everyday to free up disk space (the gc_grace is set to 600 seconds).
However, every time we run the major compaction, only 1 or 2GB disk space is freed. We tried to delete most of the data before running compaction, however, the result is pretty much the same.
So, we tried to check the source code. It seems that the column tombstones could only be purged when the row key is not in other sstables. I know the major compaction should include all sstables, however, in our use case, columns get inserted rapidly. This will make the cassandra flush the memtables to disk and create new sstables. The newly created sstables will have the same keys as the sstables that are being compacted (the compaction will take 2 or 3 hours to finish). My question is that will these newly created sstables be the cause of why most of the column-tombstone not being purged?
p.s. We also did some other tests. We inserted data to the same CF with the same wide-row pattern and deleted most of the data. This time we stopped all the writes to cassandra and did the compaction. The disk usage decreased dramatically.