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The log file contents are a sequence of 32KB blocks. The only
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exception is that 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[]
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length: uint16
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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
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won't fit). Any leftover bytes here form the trailer, which must
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consist entirely of zero 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
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non-zero length record is added, the writer must emit a FIRST record
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(which contains zero bytes of user data) to fill up the trailing seven
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bytes of the block and then emit all of the user data in subsequent
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blocks.
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More types may be added in the future. Some Readers may skip record
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types they do not understand, others may report that some data was
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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
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split into multiple fragments (typically because of block boundaries).
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FIRST is the type of the first fragment of a user record, LAST is the
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type of the last fragment of a user record, and MID is the type of all
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interior fragments of a user 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
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of the first block, second fragment occupies the entirety of the
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second block, and the third fragment occupies a prefix of the third
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block. This will leave six bytes free in the third block, which will
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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
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block boundary and scan. If there is a corruption, skip to the next
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block. As a side-benefit, we do not get confused when part of the
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contents of one log file are embedded as a record inside another log
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file.
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(2) Splitting at approximate boundaries (e.g., for mapreduce) is
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simple: find the next block boundary and skip records until we
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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
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record type, so it is a shortcoming of the current implementation,
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not necessarily the format.
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(2) No compression. Again, this could be fixed by adding new record types.
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