storage-format

Original：🇺🇸 English
Translated

SQLite file format, B-trees, pages, cells, overflow, freelist that is used in tursodb

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NPX Install

npx skill4agent add tursodatabase/turso storage-format

Tags

Translated version includes tags in frontmatter
sqlite-file-formatb-tree-storageturso-db-implementationdatabase-page-managementoverflow-pages

SKILL.md Content

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Storage Format Guide

Database File Structure

┌─────────────────────────────┐
│ Page 1: Header + Schema     │  ← First 100 bytes = DB header
├─────────────────────────────┤
│ Page 2..N: B-tree pages     │  ← Tables and indexes
│            Overflow pages   │
│            Freelist pages   │
└─────────────────────────────┘
Page size: power of 2, 512-65536 bytes. Default 4096.

Database Header (First 100 Bytes)

OffsetSizeField
016Magic: 
"SQLite format 3\0"
162Page size (big-endian)
181Write format version (1=rollback, 2=WAL)
191Read format version
244Change counter
284Database size in pages
324First freelist trunk page
364Total freelist pages
404Schema cookie
564Text encoding (1=UTF8, 2=UTF16LE, 3=UTF16BE)
All multi-byte integers: big-endian.

Page Types

FlagTypePurpose
0x02Interior indexIndex B-tree internal node
0x05Interior tableTable B-tree internal node
0x0aLeaf indexIndex B-tree leaf
0x0dLeaf tableTable B-tree leaf
-OverflowPayload exceeding cell capacity
-FreelistUnused pages (trunk or leaf)

B-tree Structure

Two B-tree types:
  • Table B-tree: 64-bit rowid keys, stores row data
  • Index B-tree: Arbitrary keys (index columns + rowid)
Interior page:  [ptr0] key1 [ptr1] key2 [ptr2] ...
                   │         │         │
                   ▼         ▼         ▼
               child     child     child
               pages     pages     pages

Leaf page:     key1:data  key2:data  key3:data ...
Page 1 always root of 
sqlite_schema
table.

Cell Format

Table Leaf Cell

[payload_size: varint] [rowid: varint] [payload] [overflow_ptr: u32?]

Table Interior Cell

[left_child_page: u32] [rowid: varint]

Index Cells

Similar but key is arbitrary (columns + rowid), not just rowid.

Record Format (Payload)

[header_size: varint] [type1: varint] [type2: varint] ... [data1] [data2] ...
Serial types:
TypeMeaning
0NULL
1-41/2/3/4 byte signed int
56 byte signed int
68 byte signed int
7IEEE 754 float
8Integer 0
9Integer 1
≥12 evenBLOB, length=(N-12)/2
≥13 oddText, length=(N-13)/2

Overflow Pages

When payload exceeds threshold, excess stored in overflow chain:
[next_page: u32] [data...]
Last page has next_page=0.

Freelist

Linked list of trunk pages, each containing leaf page numbers:
Trunk: [next_trunk: u32] [leaf_count: u32] [leaf_pages: u32...]

Turso Implementation

Key files:
  • core/storage/sqlite3_ondisk.rs
    - On-disk format, 
    PageType
    enum
  • core/storage/btree.rs
    - B-tree operations (large file)
  • core/storage/pager.rs
    - Page management
  • core/storage/buffer_pool.rs
    - Page caching

Debugging Storage

bash
# Integrity check
cargo run --bin tursodb test.db "PRAGMA integrity_check;"

# Page count
cargo run --bin tursodb test.db "PRAGMA page_count;"

# Freelist info
cargo run --bin tursodb test.db "PRAGMA freelist_count;"

References