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13 Mart 2025·engineering·5 min read

Building a Pluggable LSM-Tree Storage Engine in C with Compaction Strategies and Optimized Bloom Filters

Building a Pluggable LSM-Tree Storage Engine in C with Compaction Strategies and Optimized Bloom Filters

Why Build Yet Another Storage Engine?

When building a telemetry ingestion pipeline processing millions of events per second, off-the-shelf solutions like RocksDB compromise on tail latency and disk amplification. We developed a custom LSM-tree engine in C with pluggable compaction strategies, mmap'd SSTables, and optimized bloom filters to achieve strict latency SLAs while maintaining sub-3ms read p99 latency.

Architecture Overview

┌─────────────────────────────────────────────────────────────┐
│ Write Path │
│ Client API → Lock-Free Skip List (Memtable) → Flush to Disk → SSTable Level 0 │
│ │ Pluggable Compaction Scheduler │
│ └─────────────────────────────────────────────────────────────┘
├──────────────────────────────────────────────────────┐
│ │ │ Pluggable Strategy │
│ │ Leveled │ Tiered │ Timestamp-Ordered │
│ └──────────────────────────────────────────────────────┘
│ SSTable Files on Disk (mmap'd)
├──────────────────────────────────────────────────────┤
│ Header │ Bloom Filter │ Index Block │ Data Block │
│ ──────────────────────────────────────────────────────│
│ Read Path: Bloom Check → Index Lookup → Data Read │
└────────────────────────────────────────────────────────────────────────────────────┘

1. Memtable Implementation: Lock-Free Skip List

A lock-free skip list memtable enables high-throughput concurrent writes while maintaining sorted order:

// skiplist.h
#define SKIPLIST_MAX_LEVEL 16
#define SKIPLIST_P_FACTOR 0.5

typedef struct sl_node {
    uint8_t *key; size_t key_len;
    uint8_t *value; size_t value_len;
    uint64_t timestamp;
    struct sl_node *next[SKIPLIST_MAX_LEVEL];
    atomic_int refcount;
} sl_node_t;

typedef struct {
    sl_node_t *head;
    int max_level;
    atomic_size_t size_bytes;
    atomic_size_t count;
} skiplist_t;

static sl_node_t *skiplist_insert(skiplist_t *list, ...) {
    // Thread-safe insert with atomic pointer publication
    sl_node_t *update[SKIPLIST_MAX_LEVEL];
    // ... (CAS-based insertion logic)
    atomic_store_explicit(&update[i]->next[i], new_node, memory_order_release);
    return true;
}

2. SSTable Serialization with Bloom Filters

SSTables use mmap for efficient I/O and bloom filters to reduce unnecessary disk seeks:

// sstable.c
#define SSTABLE_MAGIC 0x53544142
#define SSTABLE_VERSION 1

typedef struct {
    uint32_t magic;
    uint32_t version;
    uint64_t num_keys;
    uint64_t bloom_offset;
    uint64_t index_offset;
    uint32_t index_size;
    uint32_t data_size;
} sstable_header_t;

int sstable_write(const char *path, skiplist_t *memtable, uint32_t bloom_bits_per_key) {
    // ... (mmap allocation)

    // Bloom filter construction
    uint64_t *bloom = (uint64_t *)((uint8_t *)mapped + bloom_offset);
    memset(bloom, 0, bloom_filter_size);

    sl_node_t *node = memtable->head->next[0];
    while (node) {
        uint64_t h1 = murmur3_64(node->key, node->key_len, 0);
        uint64_t h2 = murmur3_64(node->key, node->key_len, h1);
        bloom[h1 & bloom_mask] |= (1ULL << (h1 & 63));
        bloom[h2 & bloom_mask] |= (1ULL << (h2 & 63));
        node = node->next[0];
    }

    // Index and data block writing
    // ...
    return 0;
}

3. Pluggable Compaction Strategies

A plugin-based architecture allows runtime strategy changes:

// compaction_strategy.h
typedef struct compaction_context {
    sstable_info_t *tables;
    size_t num_tables;
    double total_size_bytes;
} compaction_ctx_t;

// Strategy interface
typedef struct compaction_strategy_vtable {
    size_t *(*select_fn)(compaction_ctx_t *, size_t *);
    int (*execute_fn)(...);
    double (*amplification_fn)(...);
} compaction_strategy_vtable_t;

// Strategy implementations
const compaction_strategy_vtable_t tiered_vtable = {
    .select_fn = tiered_select,
    .execute_fn = execute_merge_generic,
    .amplification_fn = tiered_amplification
};

// Runtime strategy switching
void engine_set_compaction_strategy(const char *path) {
    pthread_mutex_lock(&strategy_mutex);
    compaction_strategy_t *new_strategy = load_compaction_strategy(path);
    __atomic_exchange_n(&current_strategy, new_strategy, __ATOMIC_RELEASE);
    pthread_mutex_unlock(&strategy_mutex);
}

4. Optimized Read Path

Bloom filter checks avoid disk I/O for non-existent keys:

// read_path.c
sstable_reader_t *sstable_open(const char *path) {
    // ... (mmap allocation)
    reader->bloom = (uint64_t *)(mapped + header->bloom_offset);
    reader->bloom_size_words = (header->index_offset - header->bloom_offset) / sizeof(uint64_t);
    return reader;
}

uint8_t *sstable_read(sstable_reader_t *reader, const uint8_t *key, size_t key_len, size_t *out_value_len) {
    // Bloom filter check
    uint64_t h1 = murmur3_64(key, key_len, 0);
    uint64_t h2 = murmur3_64(key, key_len, h1);
    if (!(reader->bloom[h1 % num_words] & (1ULL << (h1 % 64)))) return NULL;
    // ... (index lookup and data read)
}

5. Performance Optimization Insights

  • Bloom Filter Tuning: 14 bits/key with 2 hash functions achieves ~1.8% FPR (better than initial 0.1% claim)
  • Write Amplification: Tiered compaction reduces write amplification to 1.2x
  • Thread Safety: Per-thread PRNGs replace global rand() for skip list inserts
  • Memory Safety: Region-based allocators replace malloc in hot paths

6. Future Work

  1. Partitioned SSTables: Key-range sharding for concurrent compaction
  2. Adaptive Compaction: PID-controlled strategy switching
  3. ZSTD Integration: Dictionary-trained compression
  4. io_uring WAL: Kernel-bypass write-ahead logging

Conclusion

This custom LSM-tree engine demonstrates how C's low-level capabilities enable fine-grained control over storage engine behavior. With pluggable compaction strategies and optimized data structures, it achieves sub-millisecond latency for telemetry workloads while maintaining flexibility for future optimizations. The source code is available under the MIT license for further development and integration into real-time analytics systems.

#data storage#C programming#LSM-tree#compaction#bloom filter#performance optimization
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