USE WITH CAUTION: Pebble is a work-in-progress.

Pebble

Pebble is a LevelDB/RocksDB inspired key-value store focused on performance and internal usage by CockroachDB. Pebble inherits the RocksDB file formats and a few extensions such as range deletion tombstones, table-level bloom filters, and updates to the MANIFEST format.

Pebble intentionally does not aspire to include every feature in RocksDB and is specifically targetting the use case and feature set needed by CockroachDB:

• Block-based tables
• Checkpoints
• Indexed batches
• Iterator options (lower/upper bound, table filter)
• Level-based compaction
• Manual compaction
• Merge operator
• Prefix bloom filters
• Prefix iteration
• Range deletion tombstones
• Reverse iteration
• SSTable ingestion
• Single delete
• Snapshots
• Table-level bloom filters

RocksDB has a large number of features that are not implemented in Pebble:

• Backups
• Column families
• Delete files in range
• FIFO compaction style
• Forward iterator / tailing iterator
• Hash table format
• Memtable bloom filter
• Persistent cache
• Pin iterator key / value
• Plain table format
• SSTable ingest-behind
• Sub-compactions
• Transactions
• Universal compaction style

Pebble may silently corrupt data or behave incorrectly if used with a RocksDB database that uses a feature Pebble doesn't support. Caveat emptor!

Advantages

Pebble offers several improvements over RocksDB:

• Faster reverse iteration via backwards links in the memtable's skiplist.
• Faster commit pipeline that achieves better concurrency.
• Seamless merged iteration of indexed batches. The mutations in the batch conceptually occupy another memtable level.
• Smaller, more approachable code base.
• Pacing of flushes vs compactions: Pebble smooths latency spikes caused by flushes and compactions by only flushing/compacting as fast as necessary to keep up with user writes.

See the Pebble vs RocksDB: Implementation Differences doc for more details on implementation differences.

RocksDB Compatibility

Pebble strives for forward compatibility with RocksDB 6.2.1 (the latest version of RocksDB used by CockroachDB). Forward compatibility means that a DB generated by RocksDB can be used by Pebble. Currently, Pebble provides bidirectional compatibility with RocksDB (a Pebble generated DB can be used by RocksDB), but that will change in the future as new functionality is introduced to Pebble. In general, Pebble only provides compatibility with the subset of functionality and configuration used by CockroachDB. The scope of RocksDB functionality and configuration is too large to adequately test and document all the incompatibilities. The list below contains known incompatibilities.

• Pebble's use of WAL recycling is only compatible with RocksDB's kTolerateCorruptedTailRecords WAL recovery mode. Older versions of RocksDB would automatically map incompatible WAL recovery modes to kTolerateCorruptedTailRecords. New versions of RocksDB will disable WAL recycling.
• Column families. Pebble does not support column families, nor does it attempt to detect their usage when opening a DB that may contain them.
• Hash table format. Pebble does not support the hash table sstable format.
• Plain table format. Pebble does not support the plain table sstable format.
• SSTable format version 3 and 4. Pebble does not currently support version 3 and version 4 format sstables. The sstable format version is controlled by the BlockBasedTableOptions::format_version option. See #97.

Pedigree

Pebble is based on the incomplete Go version of LevelDB:

https://github.com/golang/leveldb

The Go version of LevelDB is based on the C++ original:

https://github.com/google/leveldb

Optimizations and inspiration were drawn from RocksDB:

https://github.com/facebook/rocksdb