LSM trees do not need write-ahead log in general case:
- When new data arrives, it is converted to SSTable, which is then stored to disk in an atomic manner before returning 'success' to the client, who writes the data. If computer crashes in the middle of write, no problems - the partially written SSTable will be dropped on database start, since it isn't registered in the database yet.
- When computer crashes in the middle of background merge of smaller SSTables into bigger ones, then no problem - the source SSTables are still available after database restart, while partially written output SSTable can be safely dropped on database restart.
VictoriaMetrics and VictoriaLogs use LSM trees without WAL, while providing good durability levels. They can lose recently ingested metrics or logs on server crash, if they weren't converted to SSTables and weren't written to disk yet. But this is very good tradeoff comparing to data corruption or failed WAL replay in other systems, which use WAL in improper ways - https://valyala.medium.com/wal-usage-looks-broken-in-modern-... .
- When new data arrives, it is converted to SSTable, which is then stored to disk in an atomic manner before returning 'success' to the client, who writes the data. If computer crashes in the middle of write, no problems - the partially written SSTable will be dropped on database start, since it isn't registered in the database yet.
- When computer crashes in the middle of background merge of smaller SSTables into bigger ones, then no problem - the source SSTables are still available after database restart, while partially written output SSTable can be safely dropped on database restart.
VictoriaMetrics and VictoriaLogs use LSM trees without WAL, while providing good durability levels. They can lose recently ingested metrics or logs on server crash, if they weren't converted to SSTables and weren't written to disk yet. But this is very good tradeoff comparing to data corruption or failed WAL replay in other systems, which use WAL in improper ways - https://valyala.medium.com/wal-usage-looks-broken-in-modern-... .