Leveled Compaction Strategy

The idea of LeveledCompactionStrategy (LCS) is that all sstables are put into different levels where we guarantee that no overlapping sstables are in the same level. By overlapping we mean that the first/last token of a single sstable are never overlapping with other sstables. This means that for a SELECT we will only have to look for the partition key in a single sstable per level. Each level is 10x the size of the previous one and each sstable is 160MB by default. L0 is where sstables are streamed/flushed - no overlap guarantees are given here.

When picking compaction candidates we have to make sure that the compaction does not create overlap in the target level. This is done by always including all overlapping sstables in the next level. For example if we select an sstable in L3, we need to guarantee that we pick all overlapping sstables in L4 and make sure that no currently ongoing compactions will create overlap if we start that compaction. We can start many parallel compactions in a level if we guarantee that we wont create overlap. For L0 -> L1 compactions we almost always need to include all L1 sstables since most L0 sstables cover the full range. We also can’t compact all L0 sstables with all L1 sstables in a single compaction since that can use too much memory.

When deciding which level to compact LCS checks the higher levels first (with LCS, a “higher” level is one with a higher number, L0 being the lowest one) and if the level is behind a compaction will be started in that level.

Major compaction

It is possible to do a major compaction with LCS - it will currently start by filling out L1 and then once L1 is full, it continues with L2 etc. This is sub optimal and will change to create all the sstables in a high level instead, CASSANDRA-11817.

Bootstrapping

During bootstrap sstables are streamed from other nodes. The level of the remote sstable is kept to avoid many compactions after the bootstrap is done. During bootstrap the new node also takes writes while it is streaming the data from a remote node - these writes are flushed to L0 like all other writes and to avoid those sstables blocking the remote sstables from going to the correct level, we only do STCS in L0 until the bootstrap is done.

STCS in L0

If LCS gets very many L0 sstables reads are going to hit all (or most) of the L0 sstables since they are likely to be overlapping. To more quickly remedy this LCS does STCS compactions in L0 if there are more than 32 sstables there. This should improve read performance more quickly compared to letting LCS do its L0 -> L1 compactions. If you keep getting too many sstables in L0 it is likely that LCS is not the best fit for your workload and STCS could work out better.

Starved sstables

If a node ends up with a leveling where there are a few very high level sstables that are not getting compacted they might make it impossible for lower levels to drop tombstones etc. For example, if there are sstables in L6 but there is only enough data to actually get a L4 on the node the left over sstables in L6 will get starved and not compacted. This can happen if a user changes sstable_size_in_mb from 5MB to 160MB for example. To avoid this LCS tries to include those starved high level sstables in other compactions if there has been 25 compaction rounds where the highest level has not been involved.

LCS options

sstable_size_in_mb (default: 160MB)
The target compressed (if using compression) sstable size - the sstables can end up being larger if there are very large partitions on the node.
fanout_size (default: 10)
The target size of levels increases by this fanout_size multiplier. You can reduce the space amplification by tuning this option.

LCS also support the cassandra.disable_stcs_in_l0 startup option (-Dcassandra.disable_stcs_in_l0=true) to avoid doing STCS in L0.