| = Size Tiered Compaction Strategy |
| |
| [[stcs]] |
| The basic idea of `SizeTieredCompactionStrategy` (STCS) is to merge |
| sstables of approximately the same size. All sstables are put in |
| different buckets depending on their size. An sstable is added to the |
| bucket if size of the sstable is within `bucket_low` and `bucket_high` |
| of the current average size of the sstables already in the bucket. This |
| will create several buckets and the most interesting of those buckets |
| will be compacted. The most interesting one is decided by figuring out |
| which bucket's sstables takes the most reads. |
| |
| == Major compaction |
| |
| When running a major compaction with STCS you will end up with two |
| sstables per data directory (one for repaired data and one for |
| unrepaired data). There is also an option (-s) to do a major compaction |
| that splits the output into several sstables. The sizes of the sstables |
| are approximately 50%, 25%, 12.5%... of the total size. |
| |
| [[stcs_options]] |
| == STCS options |
| |
| `min_sstable_size` (default: 50MB):: |
| Sstables smaller than this are put in the same bucket. |
| `bucket_low` (default: 0.5):: |
| How much smaller than the average size of a bucket a sstable should be |
| before not being included in the bucket. That is, if |
| `bucket_low * avg_bucket_size < sstable_size` (and the `bucket_high` |
| condition holds, see below), then the sstable is added to the bucket. |
| `bucket_high` (default: 1.5):: |
| How much bigger than the average size of a bucket a sstable should be |
| before not being included in the bucket. That is, if |
| `sstable_size < bucket_high * avg_bucket_size` (and the `bucket_low` |
| condition holds, see above), then the sstable is added to the bucket. |
| |
| == Defragmentation |
| |
| Defragmentation is done when many sstables are touched during a read. |
| The result of the read is put in to the memtable so that the next read |
| will not have to touch as many sstables. This can cause writes on a |
| read-only-cluster. |
