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Pebble ^_{_Coverage}

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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 specifically targets 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

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

Production Ready

Pebble was introduced as an alternative storage engine to RocksDB in CockroachDB v20.1 (released May 2020) and was used in production successfully at that time. Pebble was made the default storage engine in CockroachDB v20.2 (released Nov 2020). Pebble is being used in production by users of CockroachDB at scale and is considered stable and production ready.

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.
• L0 sublevels and flush splitting for concurrent compactions out of L0 and reduced read-amplification during heavy write load.
• Faster LSM edits in LSMs with large numbers of sstables through use of a copy-on-write B-tree to hold file metadata.
• Delete-only compactions that drop whole sstables that fall within the bounds of a range deletion.
• Block-property collectors and filters that enable iterators to skip tables, index blocks and data blocks that are irrelevant, according to user-defined properties over key-value pairs.
• Range keys API, allowing KV pairs defined over a range of keyspace with user-defined semantics and interleaved during iteration.
• Smaller, more approachable code base.

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 6.2.1 can be upgraded for use by Pebble. Pebble versions in the v1 series may open DBs generated by RocksDB 6.2.1. Since its introduction, Pebble has adopted various backwards-incompatible format changes that are gated behind new 'format major versions'. The Pebble master branch does not support opening DBs generated by RocksDB. DBs generated by RocksDB may only be used with recent versions of Pebble after migrating them through format major version upgrades using previous versions of Pebble. See the below section of format major versions.

Even the RocksDB-compatible versions of Pebble only provide 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 support version 3 and version 4 format sstables. The sstable format version is controlled by the BlockBasedTableOptions::format_version option. See #97.

Format major versions

Over time Pebble has introduced new physical file formats. Backwards incompatible changes are made through the introduction of 'format major versions'. By default, when Pebble opens a database, it defaults to the lowest supported version. In v1, this is FormatMostCompatible, which is bi-directionally compatible with RocksDB 6.2.1 (with the caveats described above).

Databases created by RocksDB or Pebble versions v1 and earlier must be upgraded to a compatible format major version before running newer Pebble versions. Newer Pebble versions will refuse to open databases in no longer supported formats.

To opt into new formats, a user may set FormatMajorVersion on the Options supplied to Open, or upgrade the format major version at runtime using DB.RatchetFormatMajorVersion. Format major version upgrades are permanent; There is no option to return to an earlier format.

The table below outlines the history of format major versions, along with what range of Pebble versions support that format.

Upgrading to a format major version with 'Background' in the migration column may trigger background activity to rewrite physical file formats, typically through compactions. Upgrading to a format major version with 'Blocking' in the migration column will block until a migration is complete. The database may continue to serve reads and writes if upgrading a live database through RatchetFormatMajorVersion, but the method call will not return until the migration is complete.

For reference, the table below lists the range of supported Pebble format major versions for CockroachDB releases.

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

Getting Started

Example Code

package main

import (
	"fmt"
	"log"

	"github.com/cockroachdb/pebble"
)

func main() {
	db, err := pebble.Open("demo", &pebble.Options{})
	if err != nil {
		log.Fatal(err)
	}
	key := []byte("hello")
	if err := db.Set(key, []byte("world"), pebble.Sync); err != nil {
		log.Fatal(err)
	}
	value, closer, err := db.Get(key)
	if err != nil {
		log.Fatal(err)
	}
	fmt.Printf("%s %s\n", key, value)
	if err := closer.Close(); err != nil {
		log.Fatal(err)
	}
	if err := db.Close(); err != nil {
		log.Fatal(err)
	}
}