About a year ago, we did a major compaction in our cassandra cluster (a n00b mistake, I know), and since then we've had huge sstables that never get compacted, and we were condemned to repeat the major compaction process every once in a while (we are using SizeTieredCompaction strategy, and we've not avaluated yet LeveledCompaction, because it has its downsides, and we've had no time to test all of them in our environment).

I was trying to find a way to solve this situation (that is, do something like a major compaction that writes small sstables, not huge as major compaction does), and I couldn't find it in the documentation. I tried cleanup and scrub/upgradesstables, but they don't do that (as documentation states). Then I tried deleting all data in a node and then bootstrapping it (or "nodetool rebuild"-ing it), hoping that this way the sstables would get cleaned from deleted records and updates. But the deleted node just copied the sstables from another node as they were, cleaning nothing. 

So I tried a new approach: I switched the sstable compaction strategy (SizeTiered to Leveled), forcing the sstables to be rewritten from scratch, and then switching it back (Leveled to SizeTiered). It took a while (but so do the major compaction process) and it worked, I have smaller sstables, and I've regained a lot of disk space.

I'm happy with the results, but it doesn't seem a orthodox way of "cleaning" the sstables. What do you think, is it something wrong or crazy? Is there a different way to achieve the same thing?

Let's put an example:
Suppose you have a write-only columnfamily (no updates and no deletes, so no need for LeveledCompaction, because SizeTiered works perfectly and requires less I/O) and you mistakenly run a major compaction on it. After a few months you need more space and you delete half the data, and you find out that you're not freeing half the disk space, because most of those records were in the "major compacted" sstables. How can you free the disk space? Waiting will do you no good, because the huge sstable won't get compacted anytime soon. You can run another major compaction, but that would just postpone the real problem. Then you can switch compaction strategy and switch it back, as I just did. Is there any other way?

Tomàs Núñez
Tel. + 34 93 159 31 00 
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Skype: tomas.nunez.groupalia
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