# 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 <data> │ checksum <4 byte> │
└───────────────┴───────────────────┴─────────────┴───────────────────┘
```

Notes:

* `len`: Chunk size in bytes. 1 to 5 bytes long using the [`<uvarint>` encoding](https://go.dev/src/encoding/binary/varint.go).
* `encoding`: Currently one of `XOR`, `XOR2`, `histogram`, `floathistogram`, `histogramST`, or `floathistogramST`, see [code for numerical values](https://github.com/prometheus/prometheus/blob/02d0de9987ad99dee5de21853715954fadb3239f/tsdb/chunkenc/chunk.go#L28-L47). The `XOR2`, `histogramST`, and `floathistogramST` encodings extend their non-ST counterparts with optional Start Timestamp (ST) data. `XOR2` is gated behind the experimental [`xor2-encoding` feature flag](../../../docs/feature_flags.md#xor2-chunk-encoding); `histogramST` and `floathistogramST` are gated behind the experimental [`histograms-st-encoding` feature flag](../../../docs/feature_flags.md#histogram-st-chunk-encoding).
* `data`: See below for each encoding.
* `checksum`: Checksum of `encoding` and `data`. It's a [cyclic redundancy check](https://en.wikipedia.org/wiki/Cyclic_redundancy_check) with the Castagnoli polynomial, serialised as an unsigned 32 bits big endian number. Can be referred as a `CRC-32C`.

## XOR chunk data

```
┌──────────────────────┬───────────────┬───────────────┬──────────────────────┬──────────────────────┬──────────────────────┬──────────────────────┬─────┬──────────────────────┬──────────────────────┬──────────────────┐
│ num_samples <uint16> │ ts_0 <varint> │ v_0 <float64> │ ts_1_delta <uvarint> │ v_1_xor <varbit_xor> │ ts_2_dod <varbit_ts> │ v_2_xor <varbit_xor> │ ... │ ts_n_dod <varbit_ts> │ v_n_xor <varbit_xor> │ padding <x bits> │
└──────────────────────┴───────────────┴───────────────┴──────────────────────┴──────────────────────┴──────────────────────┴──────────────────────┴─────┴──────────────────────┴──────────────────────┴──────────────────┘
```

### 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](https://github.com/prometheus/prometheus/blob/7309c20e7e5774e7838f183ec97c65baa4362edc/tsdb/chunkenc/xor.go#L220-L253).
* `<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](https://github.com/prometheus/prometheus/blob/7309c20e7e5774e7838f183ec97c65baa4362edc/tsdb/chunkenc/xor.go#L179-L205).
* `padding` of 0 to 7 bits so that the whole chunk data is byte-aligned.
* The chunk can have as few as one sample, i.e. `ts_1`, `v_1`, etc. are optional.

## XOR2 chunk data

XOR2 uses the same structure as XOR for samples 0 and 1. Starting from sample 2,
a joint control prefix encodes both the timestamp delta-of-delta (dod) and whether
the value changed, with common dod cases byte-aligned for efficient writing.

XOR2 can encode start timestamp (ST) as well optionally, see details further
down.


```
┌──────────────────────┬───────────────────┬───────────────┬───────────────┬────────────────┬─-
│ num_samples <uint16> │ st_header <uint8> | ts_0 <varint> │ v_0 <float64> │ ?st_0 <varint> |
└──────────────────────┴───────────────────┴───────────────┴───────────────┴────────────────┴─-

-─────────────────────┬───────────────────────┬─────────────────────────┬─-
 ts_1_delta <uvarint> │ v_1_xor <varbit_xor2> │ ?st_1_delta <varbit_ts> |
-─────────────────────┴───────────────────────┴─────────────────────────┴─-

-─────────────────────────┬───────────────────────┬─────┬─-
 sample_2 <joint_sample2> │ ?st_2_dod <varbit_ts> | ... │
-─────────────────────────┴───────────────────────┴─────┴─-

-─────────────────────────┬───────────────────────┬──────────────────┐
 sample_n <joint_sample2> │ ?st_n_dod <varbit_ts> | padding <x bits> │
-─────────────────────────┴───────────────────────┴──────────────────┘

```

### Joint sample encoding for n >= 2 (`<joint_sample2>`):

Each sample starts with a variable-length control prefix that jointly encodes the
dod and value change status:

| Control prefix | dod | Value encoding that follows |
|---|---|---|
| `0` | 0 | (none, value unchanged) |
| `10` | 0 | `<varbit_xor2_nn>` (value known non-zero and non-stale) |
| `110DDDDD` `DDDDDDDD` | 13-bit signed [-4096, 4095] | `<varbit_xor2>` |
| `1110DDDD` `DDDDDDDD` `DDDDDDDD` | 20-bit signed [-524288, 524287] | `<varbit_xor2>` |
| `11110` + 64-bit dod | exact | `<varbit_xor2>` |
| `11111` | 0 | (none, stale NaN — no value field) |

The `110` and `1110` cases pack the prefix and the most-significant dod bits into
the first byte, making the full dod field byte-aligned.

### Value delta encoding (`<varbit_xor2>`):

Used after the dod≠0 control prefixes. The XOR of the current and previous value is encoded as:

| Prefix | Meaning |
|---|---|
| `0` | XOR = 0 (value unchanged) |
| `10` | Reuse previous leading/trailing window; `sigbits` value bits follow |
| `110` + leading(5) + sigbits(6) + value(sigbits) | New leading/trailing window |
| `111` | Stale NaN marker (3 bits) |

### Value delta encoding, known non-zero (`<varbit_xor2_nn>`):

Used after the `10` control prefix (dod=0, value known to have changed and be non-stale).
The delta=0 check is skipped, saving one bit on the reuse path:

| Prefix | Meaning |
|---|---|
| `0` | Reuse previous leading/trailing window; `sigbits` value bits follow |
| `1` + leading(5) + sigbits(6) + value(sigbits) | New leading/trailing window |

### Start timestamp encoding

* We use `st_i_dod` and `st_i` interchangeably when `i>1` in these notes.
* `st_header` is one byte:
   ```
   ┌───────────────────────┬───────────────────────┐
   │ first_st_known<1 bit> | st_changed_on<7 bits> │
   └───────────────────────┴───────────────────────┘
   ```
   where the highest bit `first_st_known` indicates if `st_0` is present or not.
   If the lower 7bits `st_changed_on` is 0, no `st_i (i>0)` is present.
   Otherwise `st_i (i>=st_changed_on>)` is present, while
   `st_i (0<i<st_changed_on)` is not present.

   Due to the 7 bit limitation, once a chunk has at least 127 samples,
   `st_changed_on` is set to 127 (0xEF) and the 127th and further samples will
   have `st_i` present.
* `st_0` is encoded as a `varint` if present.
* `st_1` is encoded as a `varbit_ts` delta from `st_0` (or from 0 if `st_0` is
   not present).
* `st_i_dod` aka `st_i (i>1)` is encoded as a `varbit_ts` "delta of delta" from
  `st_i-1` (or from 0 if `st_i-1` is not present).

## Histogram chunk data

```
┌──────────────────────┬──────────────────────────┬───────────────────────────────┬─────────────────────┬──────────────────┬──────────────────┬──────────────────────┬────────────────┬──────────────────┐
│ num_samples <uint16> │ histogram_flags <1 byte> │ zero_threshold <1 or 9 bytes> │ schema <varbit_int> │ pos_spans <data> │ neg_spans <data> │ custom_values <data> │ samples <data> │ padding <x bits> │
└──────────────────────┴──────────────────────────┴───────────────────────────────┴─────────────────────┴──────────────────┴──────────────────┴──────────────────────┴────────────────┴──────────────────┘
```

### Positive and negative spans data:

```
┌─────────────────────────┬────────────────────────┬───────────────────────┬────────────────────────┬───────────────────────┬─────┬────────────────────────┬───────────────────────┐
│ num_spans <varbit_uint> │ length_0 <varbit_uint> │ offset_0 <varbit_int> │ length_1 <varbit_uint> │ offset_1 <varbit_int> │ ... │ length_n <varbit_uint> │ offset_n <varbit_int> │
└─────────────────────────┴────────────────────────┴───────────────────────┴────────────────────────┴───────────────────────┴─────┴────────────────────────┴───────────────────────┘
```

### Custom values data:

The `custom_values` data is currently only used for schema -53 (custom bucket boundaries). For other schemas, it is empty (length of zero).

```
┌──────────────────────────┬──────────────────┬──────────────────┬─────┬──────────────────┐
│ num_values <varbit_uint> │ value_0 <custom> │ value_1 <custom> │ ... │ value_n <custom> │
└──────────────────────────┴─────────────────────────────────────┴─────┴──────────────────┘
```

### Samples data:

```
┌──────────────────────────┐
│    sample_0 <data>       │
├──────────────────────────┤
│    sample_1 <data>       │
├──────────────────────────┤
│    sample_2 <data>       │
├──────────────────────────┤
│          ...             │
├──────────────────────────┤
│    sample_n <data>       │
└──────────────────────────┘
```

#### Sample 0 data:

```
┌─────────────────┬─────────────────────┬──────────────────────────┬───────────────┬───────────────────────────┬─────┬───────────────────────────┬───────────────────────────┬─────┬───────────────────────────┐
│ ts <varbit_int> │ count <varbit_uint> │ zero_count <varbit_uint> │ sum <float64> │ pos_bucket_0 <varbit_int> │ ... │ pos_bucket_n <varbit_int> │ neg_bucket_0 <varbit_int> │ ... │ neg_bucket_n <varbit_int> │
└─────────────────┴─────────────────────┴──────────────────────────┴───────────────┴───────────────────────────┴─────┴───────────────────────────┴───────────────────────────┴─────┴───────────────────────────┘
```

#### Sample 1 data:

```
┌───────────────────────┬──────────────────────────┬───────────────────────────────┬──────────────────────┬─────────────────────────────────┬─────┬─────────────────────────────────┬─────────────────────────────────┬─────┬─────────────────────────────────┐
│ ts_delta <varbit_int> │ count_delta <varbit_int> │ zero_count_delta <varbit_int> │ sum_xor <varbit_xor> │ pos_bucket_0_delta <varbit_int> │ ... │ pos_bucket_n_delta <varbit_int> │ neg_bucket_0_delta <varbit_int> │ ... │ neg_bucket_n_delta <varbit_int> │
└───────────────────────┴──────────────────────────┴───────────────────────────────┴──────────────────────┴─────────────────────────────────┴─────┴─────────────────────────────────┴─────────────────────────────────┴─────┴─────────────────────────────────┘
```

#### Sample 2 data and following:

```
┌─────────────────────┬────────────────────────┬─────────────────────────────┬──────────────────────┬───────────────────────────────┬─────┬───────────────────────────────┬───────────────────────────────┬─────┬───────────────────────────────┐
│ ts_dod <varbit_int> │ count_dod <varbit_int> │ zero_count_dod <varbit_int> │ sum_xor <varbit_xor> │ pos_bucket_0_dod <varbit_int> │ ... │ pos_bucket_n_dod <varbit_int> │ neg_bucket_0_dod <varbit_int> │ ... │ neg_bucket_n_dod <varbit_int> │
└─────────────────────┴────────────────────────┴─────────────────────────────┴──────────────────────┴───────────────────────────────┴─────┴───────────────────────────────┴───────────────────────────────┴─────┴───────────────────────────────┘
```

### Notes:

* `histogram_flags` is a byte of which currently only the first two bits are used:
  * `10`: Counter reset between the previous chunk and this one.
  * `01`: No counter reset between the previous chunk and this one.
  * `00`: Counter reset status unknown.
  * `11`: Chunk is part of a gauge histogram, no counter resets are happening.
* `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 either -4 <= n <= 8 (standard exponential schemas) or -53
  (custom bucket boundaries).
* `<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.
  See [code for details](https://github.com/prometheus/prometheus/blob/8c1507ebaa4ca552958ffb60c2d1b21afb7150e4/tsdb/chunkenc/varbit.go#L31-L60).
* `<varbit_uint>` is a variable bitwidth encoding for unsigned integers with the same bit-bucketing as `<varbit_int>`.
  See [code for details](https://github.com/prometheus/prometheus/blob/8c1507ebaa4ca552958ffb60c2d1b21afb7150e4/tsdb/chunkenc/varbit.go#L136-L165).
* `<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](https://github.com/prometheus/prometheus/blob/8c1507ebaa4ca552958ffb60c2d1b21afb7150e4/tsdb/chunkenc/histogram.go#L538-L574).
* `padding` of 0 to 7 bits so that the whole chunk data is byte-aligned.
* Note that buckets are inherently deltas between the current bucket and the previous bucket. Only `bucket_0` is an absolute count.
* The chunk can have as few as one sample, i.e. sample 1 and following are optional.
* Similarly, there could be down to zero spans and down to zero buckets.

The `<custom>` encoding within the custom values data depends on the schema.
For schema -53 (custom bucket boundaries, currently the only use case for
custom values), the values to encode are bucket boundaries in the form of
floats. The encoding of a given float value _x_ works as follows:

1. Create an intermediate value _y_ = _x_ * 1000.
2. If 0 ≤ _y_ ≤ 33554430 _and_ if the decimal value of _y_ is integer, store
   _y_ + 1 as `<varbit_uint>`.
3. Otherwise, store a 0 bit, followed by the 64 bit of the original _x_
   encoded as plain `<float64>`.

Note that values stored as per (2) will always start with a 1 bit, which allow
decoders to recognize this case in contrast to values stores as per (3), which
always start with a 0 bit.

The rational behind this encoding is that most custom bucket boundaries are set
by humans as decimal numbers with not very many decimal places. In most cases,
the encoding will therefore result in a short varbit representation. The upper
bound of 33554430 is picked so that the varbit encoded value will take at most
4 bytes.

## Histogram ST chunk data

The histogram ST chunk extends the histogram chunk format with optional Start
Timestamp (ST) data, using the same ST encoding scheme as XOR2. The base
histogram sample encoding is identical to the
[histogram chunk](#histogram-chunk-data). The total header is 3 bytes — the
same size as the histogram chunk header — but the counter-reset bits are
relocated into the high 2 bits of the sample count byte so that byte 2 can
hold the ST header. An optional ST field is appended after each sample's data.

```
┌────────────────────────┬───────────────────────┬────────────────────┬───────────────────────────────┬─────────────────────┬──────────────────┬──────────────────┬──────────────────────┬────────────────┬──────────────────┐
│ counter_reset <2 bits> │ num_samples <14 bits> │ st_header <1 byte> │ zero_threshold <1 or 9 bytes> │ schema <varbit_int> │ pos_spans <data> │ neg_spans <data> │ custom_values <data> │ samples <data> │ padding <x bits> │
└────────────────────────┴───────────────────────┴────────────────────┴───────────────────────────────┴─────────────────────┴──────────────────┴──────────────────┴──────────────────────┴────────────────┴──────────────────┘
```

### Start timestamp encoding

The ST header is one byte:

  ```
  ┌───────────────────────┬───────────────────────┐
  │ first_st_known<1 bit> │ st_changed_on<7 bits> │
  └───────────────────────┴───────────────────────┘
  ```

where the highest bit `first_st_known` indicates if `st_0` is present or not.
If the lower 7bits `st_changed_on` is 0, no `st_i (i>0)` is present.
Otherwise `st_i (i>=st_changed_on)` is present, while
`st_i (0<i<st_changed_on)` is not present.

Once a chunk has at least 127 samples,
`st_changed_on` is set to 127 (0x7F) and the 128th and further samples will
have `st_i` present.

* `st_0` is encoded as a `<varint>` if present.
* `st_1` is encoded as a `<varbit_int>` delta from the timestamp of the
  previous sample (or from 0 if not previously set).
* `st_i` (i > 1) is encoded as a `<varbit_int>` "delta of delta" of the ST
  difference from the previous sample.

### Samples data:

Each `sample_i <data>` payload uses the exact same encoding as the equivalent
sample in the [histogram chunk](#histogram-chunk-data) (sample 0, sample 1, and
sample 2-and-following), but with an optional ST field. If an ST payload is present, 
it is appended immediately after the histogram sample payload.

```
┌─────────────────┬──────────────────────┐
│ sample_0 <data> │ ?st_0 <varint>       │
├─────────────────┼──────────────────────┤
│ sample_1 <data> │ ?st_1 <varbit_int>   │
├─────────────────┼──────────────────────┤
│ sample_2 <data> │ ?st_2 <varbit_int>   │
├─────────────────┼──────────────────────┤
│       ...       │         ...          │
├─────────────────┼──────────────────────┤
│ sample_n <data> │ ?st_n <varbit_int>   │
└─────────────────┴──────────────────────┘
```

## Float histogram chunk data

Float histograms have the same layout as histograms apart from the encoding of samples.

### Samples data:

```
┌──────────────────────────┐
│    sample_0 <data>       │
├──────────────────────────┤
│    sample_1 <data>       │
├──────────────────────────┤
│    sample_2 <data>       │
├──────────────────────────┤
│          ...             │
├──────────────────────────┤
│    sample_n <data>       │
└──────────────────────────┘
```

#### Sample 0 data:

```
┌─────────────────┬─────────────────┬──────────────────────┬───────────────┬────────────────────────┬─────┬────────────────────────┬────────────────────────┬─────┬────────────────────────┐
│ ts <varbit_int> │ count <float64> │ zero_count <float64> │ sum <float64> │ pos_bucket_0 <float64> │ ... │ pos_bucket_n <float64> │ neg_bucket_0 <float64> │ ... │ neg_bucket_n <float64> │
└─────────────────┴─────────────────┴──────────────────────┴───────────────┴────────────────────────┴─────┴────────────────────────┴────────────────────────┴─────┴────────────────────────┘
```

#### Sample 1 data:

```
┌───────────────────────┬────────────────────────┬─────────────────────────────┬──────────────────────┬───────────────────────────────┬─────┬───────────────────────────────┬───────────────────────────────┬─────┬───────────────────────────────┐
│ ts_delta <varbit_int> │ count_xor <varbit_xor> │ zero_count_xor <varbit_xor> │ sum_xor <varbit_xor> │ pos_bucket_0_xor <varbit_xor> │ ... │ pos_bucket_n_xor <varbit_xor> │ neg_bucket_0_xor <varbit_xor> │ ... │ neg_bucket_n_xor <varbit_xor> │
└───────────────────────┴────────────────────────┴─────────────────────────────┴──────────────────────┴───────────────────────────────┴─────┴───────────────────────────────┴───────────────────────────────┴─────┴───────────────────────────────┘
```

#### Sample 2 data and following:

```
┌─────────────────────┬────────────────────────┬─────────────────────────────┬──────────────────────┬───────────────────────────────┬─────┬───────────────────────────────┬───────────────────────────────┬─────┬───────────────────────────────┐
│ ts_dod <varbit_int> │ count_xor <varbit_xor> │ zero_count_xor <varbit_xor> │ sum_xor <varbit_xor> │ pos_bucket_0_xor <varbit_xor> │ ... │ pos_bucket_n_xor <varbit_xor> │ neg_bucket_0_xor <varbit_xor> │ ... │ neg_bucket_n_xor <varbit_xor> │
└─────────────────────┴────────────────────────┴─────────────────────────────┴──────────────────────┴───────────────────────────────┴─────┴───────────────────────────────┴───────────────────────────────┴─────┴───────────────────────────────┘
```

## Float histogram ST chunk data

The float histogram ST chunk extends the float histogram chunk format with
optional Start Timestamp (ST) data, with the same ST encoding scheme as
[Histogram ST](#histogram-st-chunk-data). The float histogram sample encoding
is unchanged. The 3-byte chunk header layout is identical to the
[Histogram ST header layout](#histogram-st-chunk-data)).

```
┌────────────────────────┬───────────────────────┬────────────────────┬───────────────────────────────┬─────────────────────┬──────────────────┬──────────────────┬──────────────────────┬────────────────┬──────────────────┐
│ counter_reset <2 bits> │ num_samples <14 bits> │ st_header <1 byte> │ zero_threshold <1 or 9 bytes> │ schema <varbit_int> │ pos_spans <data> │ neg_spans <data> │ custom_values <data> │ samples <data> │ padding <x bits> │
└────────────────────────┴───────────────────────┴────────────────────┴───────────────────────────────┴─────────────────────┴──────────────────┴──────────────────┴──────────────────────┴────────────────┴──────────────────┘
```

### Samples data:

Each `sample_i <data>` payload uses the exact same encoding as the equivalent
sample in the [float histogram chunk](#float-histogram-chunk-data) (sample 0,
sample 1, and sample 2-and-following). The optional ST field and the
`st_header` byte follow the same encoding rules as the
[histogram ST chunk](#histogram-st-chunk-data) and
[XOR2 start timestamp encoding](#start-timestamp-encoding).

```
┌─────────────────┬──────────────────────┐
│ sample_0 <data> │ ?st_0 <varint>       │
├─────────────────┼──────────────────────┤
│ sample_1 <data> │ ?st_1 <varbit_int>   │
├─────────────────┼──────────────────────┤
│ sample_2 <data> │ ?st_2 <varbit_int>   │
├─────────────────┼──────────────────────┤
│       ...       │         ...          │
├─────────────────┼──────────────────────┤
│ sample_n <data> │ ?st_n <varbit_int>   │
└─────────────────┴──────────────────────┘
```
