prometheus/tsdb/docs/format/head_chunks.md

# Head Chunks on Disk Format

The following describes the format of a chunks file, which is created in the
`chunks_head/` directory inside the data directory.

Chunks in the files are referenced from the index by uint64 composed of
in-file offset (lower 4 bytes) and segment sequence number (upper 4 bytes).

```
┌──────────────────────────────┐
│  magic(0x0130BC91) <4 byte>  │
├──────────────────────────────┤
│    version(1) <1 byte>       │
├──────────────────────────────┤
│    padding(0) <3 byte>       │
├──────────────────────────────┤
│ ┌──────────────────────────┐ │
│ │         Chunk 1          │ │
│ ├──────────────────────────┤ │
│ │          ...             │ │
│ ├──────────────────────────┤ │
│ │         Chunk N          │ │
│ └──────────────────────────┘ │
└──────────────────────────────┘
```


# Chunk

Unlike chunks in the on-disk blocks, here we additionally store the series
reference that each chunk belongs to and the mint/maxt of the chunks. This is
because we don't have an index associated with these chunks, hence this metadata
is used while replaying the chunks.

```
┌─────────────────────┬───────────────────────┬───────────────────────┬───────────────────┬───────────────┬──────────────┬────────────────┐
| series ref <8 byte> | mint <8 byte, uint64> | maxt <8 byte, uint64> | encoding <1 byte> | len <uvarint> | data <bytes> │ CRC32 <4 byte> │
└─────────────────────┴───────────────────────┴───────────────────────┴───────────────────┴───────────────┴──────────────┴────────────────┘
```

## OOO encoding

Head chunks use the highest bit of the `encoding` field to indicate whether it is out-of-order (1) or not (0). This bit is not set for chunks in the on-disk blocks.