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Diffstat (limited to 'src/leveldb/doc/log_format.txt')
| -rw-r--r-- | src/leveldb/doc/log_format.txt | 75 |
1 files changed, 0 insertions, 75 deletions
diff --git a/src/leveldb/doc/log_format.txt b/src/leveldb/doc/log_format.txt deleted file mode 100644 index 4cca5ef6ea..0000000000 --- a/src/leveldb/doc/log_format.txt +++ /dev/null @@ -1,75 +0,0 @@ -The log file contents are a sequence of 32KB blocks. The only -exception is that the tail of the file may contain a partial block. - -Each block consists of a sequence of records: - block := record* trailer? - record := - checksum: uint32 // crc32c of type and data[] ; little-endian - length: uint16 // little-endian - type: uint8 // One of FULL, FIRST, MIDDLE, LAST - data: uint8[length] - -A record never starts within the last six bytes of a block (since it -won't fit). Any leftover bytes here form the trailer, which must -consist entirely of zero bytes and must be skipped by readers. - -Aside: if exactly seven bytes are left in the current block, and a new -non-zero length record is added, the writer must emit a FIRST record -(which contains zero bytes of user data) to fill up the trailing seven -bytes of the block and then emit all of the user data in subsequent -blocks. - -More types may be added in the future. Some Readers may skip record -types they do not understand, others may report that some data was -skipped. - -FULL == 1 -FIRST == 2 -MIDDLE == 3 -LAST == 4 - -The FULL record contains the contents of an entire user record. - -FIRST, MIDDLE, LAST are types used for user records that have been -split into multiple fragments (typically because of block boundaries). -FIRST is the type of the first fragment of a user record, LAST is the -type of the last fragment of a user record, and MIDDLE is the type of -all interior fragments of a user record. - -Example: consider a sequence of user records: - A: length 1000 - B: length 97270 - C: length 8000 -A will be stored as a FULL record in the first block. - -B will be split into three fragments: first fragment occupies the rest -of the first block, second fragment occupies the entirety of the -second block, and the third fragment occupies a prefix of the third -block. This will leave six bytes free in the third block, which will -be left empty as the trailer. - -C will be stored as a FULL record in the fourth block. - -=================== - -Some benefits over the recordio format: - -(1) We do not need any heuristics for resyncing - just go to next -block boundary and scan. If there is a corruption, skip to the next -block. As a side-benefit, we do not get confused when part of the -contents of one log file are embedded as a record inside another log -file. - -(2) Splitting at approximate boundaries (e.g., for mapreduce) is -simple: find the next block boundary and skip records until we -hit a FULL or FIRST record. - -(3) We do not need extra buffering for large records. - -Some downsides compared to recordio format: - -(1) No packing of tiny records. This could be fixed by adding a new -record type, so it is a shortcoming of the current implementation, -not necessarily the format. - -(2) No compression. Again, this could be fixed by adding new record types. |