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Signed-off-by: Antoine Pultier <45740+fungiboletus@users.noreply.github.com> Signed-off-by: Antoine Pultier <45740+fungiboletus@users.noreply.github.com>
146 lines
15 KiB
Markdown
146 lines
15 KiB
Markdown
# Chunks Disk Format
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The following describes the format of a chunks file,
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which is created in the `chunks/` directory of a block.
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The maximum size per segment file is 512MiB.
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Chunks in the files are referenced from the index by uint64 composed of
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in-file offset (lower 4 bytes) and segment sequence number (upper 4 bytes).
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```
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┌──────────────────────────────┐
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│ magic(0x85BD40DD) <4 byte> │
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├──────────────────────────────┤
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│ version(1) <1 byte> │
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├──────────────────────────────┤
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│ padding(0) <3 byte> │
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├──────────────────────────────┤
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│ ┌──────────────────────────┐ │
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│ │ Chunk 1 │ │
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│ ├──────────────────────────┤ │
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│ │ ... │ │
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│ ├──────────────────────────┤ │
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│ │ Chunk N │ │
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│ └──────────────────────────┘ │
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└──────────────────────────────┘
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```
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# Chunk
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```
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┌───────────────┬───────────────────┬──────────────┬───────────────────┐
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│ len <uvarint> │ encoding <1 byte> │ data <bytes> │ checksum <4 byte> │
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└───────────────┴───────────────────┴──────────────┴───────────────────┘
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```
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Notes:
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* `len`: Chunk size in bytes. Encoded using 1 to 10 bytes, using the [`<uvarint>` encoding](https://go.dev/src/encoding/binary/varint.go).
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* `encoding`: Currently either `XOR` (1), `histogram` (2), or `float histogram` (3).
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* `data`: See below for each encoding.
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* `checksum`: Checksum of `encoding` and `data`. It's a [cyclic redudancy check](https://en.wikipedia.org/wiki/Cyclic_redundancy_check) with the Castagnoli polynomial, serialised as an unsigned 32 bits big endian number. Can be refered as a `CRC-32C`.
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## XOR chunk data
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```
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┌──────────────────────┬───────────────┬───────────────┬──────────────────────┬──────────────────────┬──────────────────────┬──────────────────────┬─────┬──────────────────────┬──────────────────────┬──────────────────┐
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│ 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> │
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└──────────────────────┴───────────────┴───────────────┴──────────────────────┴──────────────────────┴──────────────────────┴──────────────────────┴─────┴──────────────────────┴──────────────────────┴──────────────────┘
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```
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### Notes:
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* `ts` is the timestamp, `v` is the value.
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* `...` means to repeat the previous two fields as needed, with `n` starting at 2 and going up to `num_samples` – 1.
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* `<uint16>` has 2 bytes in big-endian order.
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* `<varint>` and `<uvarint>` have 1 to 10 bytes each.
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* `ts_1_delta` is `ts_1` – `ts_0`.
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* `ts_n_dod` is the “delta of deltas” of timestamps, i.e. (`ts_n` – `ts_n-1`) – (`ts_n-1` – `ts_n-2`).
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* `v_n_xor` is the result of `v_n` XOR `v_n-1`.
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* `<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.
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See [code for details](https://github.com/prometheus/prometheus/blob/7309c20e7e5774e7838f183ec97c65baa4362edc/tsdb/chunkenc/xor.go#L220-L253).
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* `<varbit_ts>` is a specific variable bitwidth encoding for the “delta of deltas” of timestamps (signed integers that are ideally small).
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It has between 1 and 68 bits.
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see [code for details](https://github.com/prometheus/prometheus/blob/7309c20e7e5774e7838f183ec97c65baa4362edc/tsdb/chunkenc/xor.go#L179-L205).
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* `padding` of 0 to 7 bits so that the whole chunk data is byte-aligned.
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* The chunk can have as few as one sample, i.e. `ts_1`, `v_1`, etc. are optional.
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## Histogram chunk data
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```
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┌──────────────────────┬──────────────────────────┬───────────────────────────────┬─────────────────────┬──────────────────┬──────────────────┬────────────────┬──────────────────┐
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│ num_samples <uint16> │ histogram_flags <1 byte> │ zero_threshold <1 or 9 bytes> │ schema <varbit_int> │ pos_spans <data> │ neg_spans <data> │ samples <data> │ padding <x bits> │
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└──────────────────────┴──────────────────────────┴───────────────────────────────┴─────────────────────┴──────────────────┴──────────────────┴────────────────┴──────────────────┘
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```
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### Positive and negative spans data:
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```
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┌─────────────────────────┬────────────────────────┬───────────────────────┬────────────────────────┬───────────────────────┬─────┬────────────────────────┬───────────────────────┐
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│ 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> │
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└─────────────────────────┴────────────────────────┴───────────────────────┴────────────────────────┴───────────────────────┴─────┴────────────────────────┴───────────────────────┘
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```
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### Samples data:
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```
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┌──────────────────────────┐
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│ sample_0 <data> │
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├──────────────────────────┤
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│ sample_1 <data> │
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├──────────────────────────┤
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│ sample_2 <data> │
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├──────────────────────────┤
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│ ... │
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├──────────────────────────┤
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│ Sample_n <data> │
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└──────────────────────────┘
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```
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#### Sample 0 data:
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```
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┌─────────────────┬─────────────────────┬──────────────────────────┬───────────────┬───────────────────────────┬─────┬───────────────────────────┬───────────────────────────┬─────┬───────────────────────────┐
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│ 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> │
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└─────────────────┴─────────────────────┴──────────────────────────┴───────────────┴───────────────────────────┴─────┴───────────────────────────┴───────────────────────────┴─────┴───────────────────────────┘
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```
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#### Sample 1 data:
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```
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┌────────────────────────┬───────────────────────────┬────────────────────────────────┬──────────────────────┬─────────────────────────────────┬─────┬─────────────────────────────────┬─────────────────────────────────┬─────┬─────────────────────────────────┐
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│ ts_delta <varbit_uint> │ count_delta <varbit_uint> │ zero_count_delta <varbit_uint> │ 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> │
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└────────────────────────┴───────────────────────────┴────────────────────────────────┴──────────────────────┴─────────────────────────────────┴─────┴─────────────────────────────────┴─────────────────────────────────┴─────┴─────────────────────────────────┘
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```
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#### Sample 2 data and following:
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```
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┌─────────────────────┬────────────────────────┬─────────────────────────────┬──────────────────────┬───────────────────────────────┬─────┬───────────────────────────────┬───────────────────────────────┬─────┬───────────────────────────────┐
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│ 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> │
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└─────────────────────┴────────────────────────┴─────────────────────────────┴──────────────────────┴───────────────────────────────┴─────┴───────────────────────────────┴───────────────────────────────┴─────┴───────────────────────────────┘
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```
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### Notes:
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* `histogram_flags` is a byte of which currently only the first two bits are used:
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* `10`: Counter reset between the previous chunk and this one.
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* `01`: No counter reset between the previous chunk and this one.
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* `00`: Counter reset status unknown.
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* `11`: Chunk is part of a gauge histogram, no counter resets are happening.
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* `zero_threshold` has a specific encoding:
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* If 0, it is a single zero byte.
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* If a power of two between 2^-243 and 2^10, it is a single byte between 1 and 254.
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* Otherwise, it is a byte with all bits set (255), followed by a float64, resulting in 9 bytes length.
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* `schema` is a specific value defined by the exposition format. Currently valid values are -4 <= n <= 8.
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* `<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.
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See [code for details](https://github.com/prometheus/prometheus/blob/8c1507ebaa4ca552958ffb60c2d1b21afb7150e4/tsdb/chunkenc/varbit.go#L31-L60).
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* `<varbit_uint>` is a variable bitwidth encoding for unsigned integers with the same bit-bucketing as `<varbit_int>`.
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See [code for details](https://github.com/prometheus/prometheus/blob/8c1507ebaa4ca552958ffb60c2d1b21afb7150e4/tsdb/chunkenc/varbit.go#L136-L165).
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* `<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.
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See [code for details](https://github.com/prometheus/prometheus/blob/8c1507ebaa4ca552958ffb60c2d1b21afb7150e4/tsdb/chunkenc/histogram.go#L538-L574).
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* `padding` of 0 to 7 bits so that the whole chunk data is byte-aligned.
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* Note that buckets are inherently deltas between the current bucket and the previous bucket. Only `bucket_0` is an absolute count.
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* The chunk can have as few as one sample, i.e. sample 1 and following are optional.
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* Similarly, there could be down to zero spans and down to zero buckets.
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