Storage Engine

Storage Engine

CommitLog

Commitlogs are an append only log of all mutations local to a Cassandra node. Any data written to Cassandra will first be written to a commit log before being written to a memtable. This provides durability in the case of unexpected shutdown. On startup, any mutations in the commit log will be applied to memtables.

All mutations write optimized by storing in commitlog segments, reducing the number of seeks needed to write to disk. Commitlog Segments are limited by the commitlog_segment_size option, once the size is reached, a new commitlog segment is created. Commitlog segments can be archived, deleted, or recycled once all its data has been flushed to SSTables. Commitlog segments are truncated when Cassandra has written data older than a certain point to the SSTables. Running “nodetool drain” before stopping Cassandra will write everything in the memtables to SSTables and remove the need to sync with the commitlogs on startup.

commitlog_segment_size: The default size is 32MiB, which is almost always fine, but if you are archiving commitlog segments (see commitlog_archiving.properties), then you probably want a finer granularity of archiving; 8 or 16 MiB is reasonable. commitlog_segment_size also determines the default value of max_mutation_size in cassandra.yaml. By default, max_mutation_size is half the size of commitlog_segment_size.

NOTE: If max_mutation_size is set explicitly then commitlog_segment_size must be set to at least twice the size of max_mutation_size.

commitlog_sync: may be either periodic or batch.
- batch: In batch mode, Cassandra won’t ack writes until the commit log has been fsynced to disk. It will wait “commitlog_sync_batch_window_in_ms” milliseconds between fsyncs. This window should be kept short because the writer threads will be unable to do extra work while waiting. You may need to increase concurrent_writes for the same reason.
  - commitlog_sync_batch_window_in_ms: Time to wait between “batch” fsyncs Default Value: 2
- periodic: In periodic mode, writes are immediately ack’ed, and the CommitLog is simply synced every “commitlog_sync_period” milliseconds.
  - commitlog_sync_period: Time to wait between “periodic” fsyncs Default Value: 10000ms

Default Value: batch

NOTE: In the event of an unexpected shutdown, Cassandra can lose up to the sync period or more if the sync is delayed. If using “batch” mode, it is recommended to store commitlogs in a separate, dedicated device.*
- commitlog_directory: This option is commented out by default When running on magnetic HDD, this should be a separate spindle than the data directories. If not set, the default directory is $CASSANDRA_HOME/data/commitlog.

Default Value: /var/lib/cassandra/commitlog

commitlog_compression: Compression to apply to the commitlog. If omitted, the commit log will be written uncompressed. LZ4, Snappy, Deflate and Zstd compressors are supported.

(Default Value: (complex option):

#   - class_name: LZ4Compressor
#     parameters:

commitlog_total_space: Total space to use for commit logs on disk.

If space gets above this value, Cassandra will flush every dirty CF in the oldest segment and remove it. So a small total commitlog space will tend to cause more flush activity on less-active columnfamilies.

The default value is the smaller of 8192, and 1/4 of the total space of the commitlog volume.

Default Value: 8192MiB

Memtables

Memtables are in-memory structures where Cassandra buffers writes. In general, there is one active memtable per table. Eventually, memtables are flushed onto disk and become immutable SSTables. This can be triggered in several ways:

The memory usage of the memtables exceeds the configured threshold (see memtable_cleanup_threshold)
The commit-log approaches its maximum size, and forces memtable flushes in order to allow commitlog segments to be freed

Memtables may be stored entirely on-heap or partially off-heap, depending on memtable_allocation_type.

SSTables

SSTables are the immutable data files that Cassandra uses for persisting data on disk.

As SSTables are flushed to disk from memtables or are streamed from other nodes, Cassandra triggers compactions which combine multiple SSTables into one. Once the new SSTable has been written, the old SSTables can be removed.

Each SSTable is comprised of multiple components stored in separate files:

Data.db

The actual data, i.e. the contents of rows.

Index.db

An index from partition keys to positions in the Data.db file. For wide partitions, this may also include an index to rows within a partition.

Summary.db

A sampling of (by default) every 128th entry in the Index.db file.

Filter.db

A Bloom Filter of the partition keys in the SSTable.

CompressionInfo.db

Metadata about the offsets and lengths of compression chunks in the Data.db file.

Statistics.db

Stores metadata about the SSTable, including information about timestamps, tombstones, clustering keys, compaction, repair, compression, TTLs, and more.

Digest.crc32

A CRC-32 digest of the Data.db file.

TOC.txt

A plain text list of the component files for the SSTable.

Within the Data.db file, rows are organized by partition. These partitions are sorted in token order (i.e. by a hash of the partition key when the default partitioner, Murmur3Partition, is used). Within a partition, rows are stored in the order of their clustering keys.

SSTables can be optionally compressed using block-based compression.

SSTable Versions

This section was created using the following gist which utilized this original source.

The version numbers, to date are:

Version 0

b (0.7.0): added version to sstable filenames
c (0.7.0): bloom filter component computes hashes over raw key bytes instead of strings
d (0.7.0): row size in data component becomes a long instead of int
e (0.7.0): stores undecorated keys in data and index components
f (0.7.0): switched bloom filter implementations in data component
g (0.8): tracks flushed-at context in metadata component

Version 1

h (1.0): tracks max client timestamp in metadata component
hb (1.0.3): records compression ration in metadata component
hc (1.0.4): records partitioner in metadata component
hd (1.0.10): includes row tombstones in maxtimestamp
he (1.1.3): includes ancestors generation in metadata component
hf (1.1.6): marker that replay position corresponds to 1.1.5+ millis-based id (see CASSANDRA-4782)
ia (1.2.0):
- column indexes are promoted to the index file
- records estimated histogram of deletion times in tombstones
- bloom filter (keys and columns) upgraded to Murmur3
ib (1.2.1): tracks min client timestamp in metadata component
ic (1.2.5): omits per-row bloom filter of column names

Version 2

ja (2.0.0):
- super columns are serialized as composites (note that there is no real format change, this is mostly a marker to know if we should expect super columns or not. We do need a major version bump however, because we should not allow streaming of super columns into this new format)
- tracks max local deletiontime in sstable metadata
- records bloom_filter_fp_chance in metadata component
- remove data size and column count from data file (CASSANDRA-4180)
- tracks max/min column values (according to comparator)
jb (2.0.1):
- switch from crc32 to adler32 for compression checksums
- checksum the compressed data
ka (2.1.0):
- new Statistics.db file format
- index summaries can be downsampled and the sampling level is persisted
- switch uncompressed checksums to adler32
- tracks presense of legacy (local and remote) counter shards
la (2.2.0): new file name format
lb (2.2.7): commit log lower bound included