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What Are SSTables?

What Are SSTables?

Section titled “What Are SSTables?”

SSTables are immutable, sorted-string table files that persist Cassandra’s in-memory data structures to disk. They pair with memtables and the write-ahead log (WAL) in an LSM-tree design: writes land in memory and log, then flush to disk as SSTables. Immutability enables concurrent readers, simple compaction, and predictable IO.

In this chapter you will learn

Section titled “In this chapter you will learn”
  • The relationship between LSM-trees, memtables, WAL, and SSTables
  • How SSTables participate in Cassandra’s read and write paths
  • How formats evolved from big → mc/mm → BTI, and why
  • Directory layout and naming conventions, including TOC.txt

Overview

Section titled “Overview”

At a high level, Cassandra batches updates in a memtable and appends them to a WAL for durability. When a memtable fills or a flush is triggered, data is written out as an immutable SSTable on disk. Because SSTables are append-only artifacts with sorted partitions, reads can navigate quickly using auxiliary components (Bloom filter, index, summary) without rewriting data in place.

Role in Cassandra Read/Write Path

Section titled “Role in Cassandra Read/Write Path”
  • Write path (implementation view):
    • Client mutation → append to WAL → update memtable (default: SkipListMemtable; TrieMemtable is an opt-in alternative — byte-ordered prefix trie with shared prefixes and CPU-core sharding)
    • Flush triggers SSTableWriter to build components:
      • Serialize rows into Data.db (optionally compressed in fixed-size chunks)
      • Emit partition digests and offsets to Index.db; build Summary.db samples
      • Construct Filter.db (Bloom) and accumulate Statistics.db
      • Write CompressionInfo.db, Digest.crc32, and TOC.txt
  • Read path (point read, implementation view):
    • Check min/max key bounds of the SSTable; outside range → skip entirely (no Bloom check needed)
    • Compute partition key digest and check Bloom (Filter.db); negative → stop
    • Use Summary.db to narrow a region of Index.db; seek to exact index entry
    • Translate to Data.db position; read aligned to compression chunk boundaries and decompress just the needed bytes

See cross-links: Chapter 4 (flush pipeline) and Chapter 10 (read path decision tree). For a quick visual of component relationships, see the diagram referenced in Chapter 2 (/cqlite/format-guide/diagrams/sstable-components, Mermaid source committed alongside).

Evolution of Formats

Section titled “Evolution of Formats”
  • big (3.x/4.x): classic multi-file layout; partition index stores digests → data offsets; promoted index used for wide partitions
  • mc/mm (4.x): iterative improvements on big; header/version flags and metadata evolve; tooling and defaults shift
  • BTI (5.0): B-Tree/Trie Indexed family; improves lookup characteristics and index layout, reducing amplification for certain patterns while preserving the multi-component model

The on-disk component set remains recognizable across versions, but metadata and index structures evolve. Chapter 17 covers BTI in detail.

Directory Layout and Naming

Section titled “Directory Layout and Naming”

SSTable file names follow {prefix}-{generation}-{format}-{Component}.db with components enumerated in TOC.txt. Below is a tiny, real TOC.txt from test_basic/simple_table:

Data.db
Statistics.db
Digest.crc32
TOC.txt
CompressionInfo.db
Filter.db
Index.db
Summary.db
Section titled “Sidebar: Version Differences (3.x/4.x)”
  • Component set is stable (Data/Index/Summary/Filter/Stats/CompressionInfo/TOC/Digest)
  • Naming differs by format tag (big, mc/mm, bti); reader/writer internals improved in 5.0
  • Some 3.x/4.x tools and flags changed defaults; this guide assumes 5.0 behavior unless noted

Key Takeaways

Section titled “Key Takeaways”
  • SSTables are immutable, sorted disk artifacts produced by memtable flushes
  • Reads follow Bloom → Summary → Index → Data (min/max key bounds checked before Bloom for range exclusion); writes never mutate existing SSTables
  • The component set is consistent across versions; 5.0 advances internal layout with BTI
  • TOC.txt is the single source of truth for which component files exist

References

Section titled “References”