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75 lines
2.7 KiB
75 lines
2.7 KiB
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[] ; 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 |
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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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