ColumnFamilyOptions

Every column family — including the default one — has its own ColumnFamilyOptions. Compression, memtable size, compaction style, the merge operator, and the comparator are all CF-scoped.

DbOptions inherits from the same base as ColumnFamilyOptions, so a lot of the same setters are available on both. The CF-scoped ones override the DB-scoped defaults for that family.

Upstream reference: include/rocksdb/options.h, Tuning Guide.

Setting up a column family

var cfOpts = new ColumnFamilyOptions()
    .SetCompression(Compression.Zstd)
    .SetWriteBufferSize(64UL * 1024 * 1024)
    .SetMaxWriteBufferNumber(4)
    .SetTargetFileSizeBase(64UL * 1024 * 1024);

var cfs = new ColumnFamilies(cfOpts);          // default CF
cfs.Add("audit", new ColumnFamilyOptions());

using var db = RocksDb.Open(
    new DbOptions().SetCreateIfMissing(true).SetCreateMissingColumnFamilies(true),
    path,
    cfs);

Memtable & flush

Setter	What it controls
`SetWriteBufferSize(ulong)`	Memtable size before flush. Larger = fewer flushes, more memory.
`SetMaxWriteBufferNumber(int)`	How many memtables can exist at once (filling + flushing).
`SetMinWriteBufferNumberToMerge(int)`	How many to merge into one SST on flush.
`SetMemtablePrefixBloomSizeRatio(double)`	Memtable Bloom filter ratio. Speeds up point lookups.

new ColumnFamilyOptions()
    .SetWriteBufferSize(128UL * 1024 * 1024)
    .SetMaxWriteBufferNumber(4)
    .SetMinWriteBufferNumberToMerge(2);

Upstream reference: Memtable.

Compaction

Setter	What it controls
`SetCompactionStyle(Compaction)`	`Level` (default), `Universal`, `Fifo`.
`SetLevelCompactionDynamicLevelBytes(bool)`	Recommended on for level-style.
`SetTargetFileSizeBase(ulong)`	SST file size at L1.
`SetMaxBytesForLevelBase(ulong)`	Total size budget at L1.
`SetNumLevels(int)`	Number of levels.
`SetDisableAutoCompactions(int)`	Disable background compactions (manual only).

Upstream reference: Leveled Compaction, Universal Compaction.

new ColumnFamilyOptions()
    .SetCompactionStyle(Compaction.Level)
    .SetLevelCompactionDynamicLevelBytes(true)
    .SetTargetFileSizeBase(64UL * 1024 * 1024);

Compression

new ColumnFamilyOptions()
    .SetCompression(Compression.Zstd);

// Per-level compression (e.g. LZ4 on hot levels, ZSTD on archival)
var perLevel = new[]
{
    Compression.No,    // L0
    Compression.Lz4,   // L1
    Compression.Lz4,   // L2
    Compression.Zstd,  // L3
    Compression.Zstd,  // L4
    Compression.Zstd,  // L5
    Compression.Zstd,  // L6 (bottom-most)
};
new ColumnFamilyOptions().SetCompressionPerLevel(perLevel, (ulong)perLevel.Length);

Available algorithms: No, Snappy, Zlib, Bzip2, Lz4, Lz4hc, Xpress, Zstd. ZSTD usually wins on ratio, LZ4 on speed.

Upstream reference: Compression.

Comparator and prefix extractor

By default RocksDB sorts keys bytewise (memcmp). Override only if you need a different ordering — and keep in mind that the comparator name is persisted and must be stable across runs.

new ColumnFamilyOptions()
    .SetComparator(myComparator);

new ColumnFamilyOptions()
    .SetPrefixExtractor(SliceTransform.CreateFixedPrefix(4));

A prefix extractor unlocks prefix-seek — a much faster scan when most queries hit one prefix bucket.

Merge operator

var op = MergeOperators.Create("StringAppend", partial, full);

new ColumnFamilyOptions().SetMergeOperator(op);

See the Merge Operator guide.

Table format

SetBlockBasedTableFactory plugs in a configured BlockBasedTableOptions:

var table = new BlockBasedTableOptions()
    .SetBlockSize(16 * 1024)
    .SetFilterPolicy(BloomFilterPolicy.Create(10))
    .SetCacheIndexAndFilterBlocks(true);

new ColumnFamilyOptions().SetBlockBasedTableFactory(table);

BlockBasedTable options reference

Optimisation shortcuts

Setter	Profile
`OptimizeForPointLookup(ulong blockCacheSizeMb)`	Optimises for key-by-key reads (hash index, larger Bloom).
`OptimizeLevelStyleCompaction(ulong memtableMemoryBudget)`	Sensible defaults for level-style.
`OptimizeUniversalStyleCompaction(ulong memtableMemoryBudget)`	Sensible defaults for universal-style.

new ColumnFamilyOptions()
    .OptimizeForPointLookup(blockCacheSizeMb: 128);

Inspecting effective options

Once the DB is open, use:

string s = db.GetProperty("rocksdb.cfstats", users);

…or the human-readable global one:

string s = db.GetProperty("rocksdb.stats");