On Fri, Dec 4, 2009 at 5:32 PM, Ramzi Rabah <[email protected]> wrote: > Starting with fresh directories with no data and trying to do simple > inserts, I could not reproduce it *sigh*. Nothing is simple :(, so I > decided to dig deeper into the code. > > I was looking at the code for compaction, and this is a very noob > concern, so please bare with me if I'm way off, this code is all new > to me. When we are doing compactions during the normal course of > cassandra, we call: > > for (List<SSTableReader> sstables : > getCompactionBuckets(ssTables_, 50L * 1024L * 1024L)) > { > if (sstables.size() < minThreshold) > { > continue; > } > other wise docompactions... > > where getCompactionBuckets puts in buckets very small files, or files > that are 0.5-1.5 of each other's sizes. It will only compact those if > they are >= minimum threshold which is 4 by default.
Exactly right. > So far so good. Now how about this scenario, I have an old entry that > I inserted long time ago and that was compacted into a 75MB file. > There are fewer 75MB files than 4. I do many deletes, and I end with 4 > extra sstable files filled with tombstones, each about 300 MB large. > These 4 files are compacted together and in the compaction code, if > the tombstone is there we don't copy it over to the new file. Now > since we did not compact the 75MB files, but we compacted the > tombstone files, doesn't that leave us with the tombstone gone, but > the data still intact in the 75MB file? Also right. Glad you had a look! :) One relatively easy fix would be to only GC the tombstones if there are no SSTables left for that CF older than the ones being compacted. (So, a "major" compaction, which compacts all SSTables and is what nodeprobe invokes, would always GC eligible tombstones.) -Jonathan
