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76 lines
2.9 KiB
Markdown
76 lines
2.9 KiB
Markdown
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leveldb Log format
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==================
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The log file contents are a sequence of 32KB blocks. The only exception is that
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the tail of the file may contain a partial block.
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Each block consists of a sequence of records:
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block := record* trailer?
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record :=
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checksum: uint32 // crc32c of type and data[] ; little-endian
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length: uint16 // little-endian
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type: uint8 // One of FULL, FIRST, MIDDLE, LAST
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data: uint8[length]
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A record never starts within the last six bytes of a block (since it won't fit).
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Any leftover bytes here form the trailer, which must consist entirely of zero
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bytes and must be skipped by readers.
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Aside: if exactly seven bytes are left in the current block, and a new non-zero
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length record is added, the writer must emit a FIRST record (which contains zero
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bytes of user data) to fill up the trailing seven bytes of the block and then
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emit all of the user data in subsequent blocks.
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More types may be added in the future. Some Readers may skip record types they
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do not understand, others may report that some data was skipped.
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FULL == 1
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FIRST == 2
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MIDDLE == 3
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LAST == 4
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The FULL record contains the contents of an entire user record.
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FIRST, MIDDLE, LAST are types used for user records that have been split into
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multiple fragments (typically because of block boundaries). FIRST is the type
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of the first fragment of a user record, LAST is the type of the last fragment of
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a user record, and MIDDLE is the type of all interior fragments of a user
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record.
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Example: consider a sequence of user records:
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A: length 1000
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B: length 97270
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C: length 8000
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**A** will be stored as a FULL record in the first block.
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**B** will be split into three fragments: first fragment occupies the rest of
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the first block, second fragment occupies the entirety of the second block, and
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the third fragment occupies a prefix of the third block. This will leave six
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bytes free in the third block, which will be left empty as the trailer.
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**C** will be stored as a FULL record in the fourth block.
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----
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## Some benefits over the recordio format:
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1. We do not need any heuristics for resyncing - just go to next block boundary
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and scan. If there is a corruption, skip to the next block. As a
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side-benefit, we do not get confused when part of the contents of one log
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file are embedded as a record inside another log file.
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2. Splitting at approximate boundaries (e.g., for mapreduce) is simple: find the
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next block boundary and skip records until we hit a FULL or FIRST record.
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3. We do not need extra buffering for large records.
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## Some downsides compared to recordio format:
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1. No packing of tiny records. This could be fixed by adding a new record type,
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so it is a shortcoming of the current implementation, not necessarily the
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format.
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2. No compression. Again, this could be fixed by adding new record types.
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