prometheus/tsdb/docs/format/chunks.md

Chunks Disk Format

The following describes the format of a chunks file, which is created in the chunks/ directory of a block. The maximum size per segment file is 512MiB.

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(0x85BD40DD) <4 byte>  │
├──────────────────────────────┤
│    version(1) <1 byte>       │
├──────────────────────────────┤
│    padding(0) <3 byte>       │
├──────────────────────────────┤
│ ┌──────────────────────────┐ │
│ │         Chunk 1          │ │
│ ├──────────────────────────┤ │
│ │          ...             │ │
│ ├──────────────────────────┤ │
│ │         Chunk N          │ │
│ └──────────────────────────┘ │
└──────────────────────────────┘

Chunk

┌───────────────┬───────────────────┬──────────────┬────────────────┐
│ len <uvarint> │ encoding <1 byte> │ data <bytes> │ CRC32 <4 byte> │
└───────────────┴───────────────────┴──────────────┴────────────────┘

Notes:

• <uvarint> has 1 to 10 bytes.
• encoding: Currently either XOR or histogram.
• data: See below for each encoding.

XOR chunk data

┌──────────────────────┬───────────────┬───────────────┬──────────────────────┬──────────────────────┬──────────────────────┬──────────────────────┬─────┐
│ num_samples <uint16> │ ts_0 <varint> │ v_0 <float64> │ ts_1_delta <uvarint> │ v_1_xor <varbit_xor> │ ts_n_dod <varbit_ts> │ v_n_xor <varbit_xor> │ ... │
└──────────────────────┴───────────────┴───────────────┴──────────────────────┴──────────────────────┴──────────────────────┴──────────────────────┴─────┘

Notes:

• ts is the timestamp, v is the value.
• ... means to repeat the previous two fields as needed, with n starting at 2 and going up to num_samples – 1.
• <uint16> has 2 bytes in big-endian order.
• <varint> and <uvarint> have 1 to 10 bytes each.
• ts_1_delta is ts_1 – ts_0.
• ts_n_dod is the “delta of deltas” of timestamps, i.e. (ts_n – ts_n-1) – (ts_n-1 – ts_n-2).
• <v_n_xor> is the result of v_n XOR v_n-1.
• <varbit_xor> is a specific variable bitwidth encoding of the result of XORing the current and the previous value. It has between 1 bit and 77 bits. See code for details.
• <varbit_ts> is a specific variable bitwidth encoding for the “delta of deltas” of timestamps (signed integers that are ideally small). It has between 1 and 68 bits. see code for details.

Histogram chunk data

┌──────────────────────┬───────────────────────────────┬─────────────────────┬──────────────────┬──────────────────┬────────────────┐
│ num_samples <uint16> │ zero_threshold <1 or 9 bytes> │ schema <varbit_int> │ pos_spans <data> │ neg_spans <data> │ samples <data> │
└──────────────────────┴───────────────────────────────┴─────────────────────┴──────────────────┴──────────────────┴────────────────┘

Positive and negative spans data:

┌───────────────────┬────────────────────────┬───────────────────────┬─────┬──────────────────────────┬─────────────────────────┐
│ num <varbit_uint> │ length_1 <varbit_uint> │ offset_1 <varbit_int> │ ... │ length_num <varbit_uint> │ offset_num <varbit_int> │
└───────────────────┴────────────────────────┴───────────────────────┴─────┴──────────────────────────┴─────────────────────────┘

Samples data:

TODO

Notes:

• zero_threshold has a specific encoding:
  • If 0, it is a single zero byte.
  • If a power of two between 2^-243 and 2^10, it is a single byte between 1 and 254.
  • Otherwise, it is a byte with all bits set (255), followed by a float64, resulting in 9 bytes length.
• schema is a specific value defined by the exposition format. Currently valid values are -4 <= n <= 8.
• <varbit_int> is a variable bitwidth encoding for signed integers, optimized for “delta of deltas” of bucket deltas. It has between 1 bit and 9 bytes.
• <varbit_uint> is a variable bitwidth encoding for unsigned integers with the same bit-bucketing as <varbit_int>.