= Leveled Compaction Strategy

[[lcs]]
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]]
== 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.
