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89 lines
6 KiB
Markdown
89 lines
6 KiB
Markdown
# WAL Disk Format
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The write ahead log operates in segments that are numbered and sequential,
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e.g. `000000`, `000001`, `000002`, etc., and are limited to 128MB by default.
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A segment is written to in pages of 32KB. Only the last page of the most recent segment
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may be partial. A WAL record is an opaque byte slice that gets split up into sub-records
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should it exceed the remaining space of the current page. Records are never split across
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segment boundaries. If a single record exceeds the default segment size, a segment with
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a larger size will be created.
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The encoding of pages is largely borrowed from [LevelDB's/RocksDB's write ahead log.][1]
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Notable deviations are that the record fragment is encoded as:
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```
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┌───────────┬──────────┬────────────┬──────────────┐
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│ type <1b> │ len <2b> │ CRC32 <4b> │ data <bytes> │
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└───────────┴──────────┴────────────┴──────────────┘
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```
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The type flag has the following states:
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* `0`: rest of page will be empty
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* `1`: a full record encoded in a single fragment
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* `2`: first fragment of a record
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* `3`: middle fragment of a record
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* `4`: final fragment of a record
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## Record encoding
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The records written to the write ahead log are encoded as follows:
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### Series records
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Series records encode the labels that identifies a series and its unique ID.
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```
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┌────────────────────────────────────────────┐
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│ type = 1 <1b> │
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├────────────────────────────────────────────┤
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│ ┌─────────┬──────────────────────────────┐ │
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│ │ id <8b> │ n = len(labels) <uvarint> │ │
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│ ├─────────┴────────────┬─────────────────┤ │
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│ │ len(str_1) <uvarint> │ str_1 <bytes> │ │
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│ ├──────────────────────┴─────────────────┤ │
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│ │ ... │ │
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│ ├───────────────────────┬────────────────┤ │
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│ │ len(str_2n) <uvarint> │ str_2n <bytes> │ │
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│ └───────────────────────┴────────────────┘ │
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│ . . . │
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└────────────────────────────────────────────┘
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```
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### Sample records
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Sample records encode samples as a list of triples `(series_id, timestamp, value)`.
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Series reference and timestamp are encoded as deltas w.r.t the first sample.
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```
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┌──────────────────────────────────────────────────────────────────┐
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│ type = 2 <1b> │
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├──────────────────────────────────────────────────────────────────┤
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│ ┌────────────────────┬───────────────────────────┬─────────────┐ │
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│ │ id <8b> │ timestamp <8b> │ value <8b> │ │
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│ └────────────────────┴───────────────────────────┴─────────────┘ │
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│ ┌────────────────────┬───────────────────────────┬─────────────┐ │
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│ │ id_delta <uvarint> │ timestamp_delta <uvarint> │ value <8b> │ │
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│ └────────────────────┴───────────────────────────┴─────────────┘ │
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│ . . . │
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└──────────────────────────────────────────────────────────────────┘
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```
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### Tombstone records
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Tombstone records encode tombstones as a list of triples `(series_id, min_time, max_time)`
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and specify an interval for which samples of a series got deleted.
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```
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┌─────────────────────────────────────────────────────┐
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│ type = 3 <1b> │
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├─────────────────────────────────────────────────────┤
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│ ┌─────────┬───────────────────┬───────────────────┐ │
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│ │ id <8b> │ min_time <varint> │ max_time <varint> │ │
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│ └─────────┴───────────────────┴───────────────────┘ │
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│ . . . │
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└─────────────────────────────────────────────────────┘
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```
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[1][https://github.com/facebook/rocksdb/wiki/Write-Ahead-Log-File-Format]
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