In reply to my own question, I'm clearly talking nonsense. SuperColumns are cleared out when empty (because really, what's the difference from the client point of view?) and so setting TTL at the Column level is plainly good enough. This was actually solved by dropping GCGraceSeconds right down to something very small - probably not appropriate for a production environment, but at least I know that the data will be deleted properly.

In a related question - is there any way to trigger a deletion of sstables which have already been compacted (SSTABLE-CLEANUP-TIMER thread)? These could potentially take up a lot of disk space, and are surely no longer being used. What triggers Cassandra to perform these deletions, and can I do it manually?

Thanks,
Andy.

On 19/01/11 11:36, Andy Burgess wrote:
Hi,

I'm trying to use the time-to-live functionality, which I've patched into Cassandra 0.6.6. My problem is that my sstables are not emptying. When my data expires, they do reduce in size, but only by a fraction of what I expected. I've come to realise that this must be because I've set the TTL on my Columns, but actually, I need the SuperColumns to expire too - I believe that my sstables remain large because I (probably) have a lot of empty SuperColumns. Unfortunately, I can't see a way to set a TTL on a SuperColumn.

Does anyone have any suggestions for what I can do here?

Actually, I've also just been playing with a few tests of "manually" deleting a SuperColumn, rather than using TTL - i.e. using Hector's Deletion with setSuper_column. Whilst this appears to work as far as making the data unavailable to the client is concerned, it only appears to reduce sstable size by about three quarters. This is in the case where I know that the sstable contains only my test inserts, which I have completely deleted. Why doesn't the sstable reduce in size to (almost) nothing?

Thanks,
Andy.

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