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authorSfan5 <sfan5@live.de>2013-09-09 22:46:18 +0200
committerSfan5 <sfan5@live.de>2013-09-09 22:50:51 +0200
commit3725179736e2b5372664163470e7ef3dc76529a4 (patch)
tree7a56c929ae75530679b3d2f7c9c02fb361760219 /src/leveldb/doc/table_format.txt
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Use system-wide LevelDB instead of bundled one
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-File format
-===========
-
- <beginning_of_file>
- [data block 1]
- [data block 2]
- ...
- [data block N]
- [meta block 1]
- ...
- [meta block K]
- [metaindex block]
- [index block]
- [Footer] (fixed size; starts at file_size - sizeof(Footer))
- <end_of_file>
-
-The file contains internal pointers. Each such pointer is called
-a BlockHandle and contains the following information:
- offset: varint64
- size: varint64
-See https://developers.google.com/protocol-buffers/docs/encoding#varints
-for an explanation of varint64 format.
-
-(1) The sequence of key/value pairs in the file are stored in sorted
-order and partitioned into a sequence of data blocks. These blocks
-come one after another at the beginning of the file. Each data block
-is formatted according to the code in block_builder.cc, and then
-optionally compressed.
-
-(2) After the data blocks we store a bunch of meta blocks. The
-supported meta block types are described below. More meta block types
-may be added in the future. Each meta block is again formatted using
-block_builder.cc and then optionally compressed.
-
-(3) A "metaindex" block. It contains one entry for every other meta
-block where the key is the name of the meta block and the value is a
-BlockHandle pointing to that meta block.
-
-(4) An "index" block. This block contains one entry per data block,
-where the key is a string >= last key in that data block and before
-the first key in the successive data block. The value is the
-BlockHandle for the data block.
-
-(6) At the very end of the file is a fixed length footer that contains
-the BlockHandle of the metaindex and index blocks as well as a magic number.
- metaindex_handle: char[p]; // Block handle for metaindex
- index_handle: char[q]; // Block handle for index
- padding: char[40-p-q]; // zeroed bytes to make fixed length
- // (40==2*BlockHandle::kMaxEncodedLength)
- magic: fixed64; // == 0xdb4775248b80fb57 (little-endian)
-
-"filter" Meta Block
--------------------
-
-If a "FilterPolicy" was specified when the database was opened, a
-filter block is stored in each table. The "metaindex" block contains
-an entry that maps from "filter.<N>" to the BlockHandle for the filter
-block where "<N>" is the string returned by the filter policy's
-"Name()" method.
-
-The filter block stores a sequence of filters, where filter i contains
-the output of FilterPolicy::CreateFilter() on all keys that are stored
-in a block whose file offset falls within the range
-
- [ i*base ... (i+1)*base-1 ]
-
-Currently, "base" is 2KB. So for example, if blocks X and Y start in
-the range [ 0KB .. 2KB-1 ], all of the keys in X and Y will be
-converted to a filter by calling FilterPolicy::CreateFilter(), and the
-resulting filter will be stored as the first filter in the filter
-block.
-
-The filter block is formatted as follows:
-
- [filter 0]
- [filter 1]
- [filter 2]
- ...
- [filter N-1]
-
- [offset of filter 0] : 4 bytes
- [offset of filter 1] : 4 bytes
- [offset of filter 2] : 4 bytes
- ...
- [offset of filter N-1] : 4 bytes
-
- [offset of beginning of offset array] : 4 bytes
- lg(base) : 1 byte
-
-The offset array at the end of the filter block allows efficient
-mapping from a data block offset to the corresponding filter.
-
-"stats" Meta Block
-------------------
-
-This meta block contains a bunch of stats. The key is the name
-of the statistic. The value contains the statistic.
-TODO(postrelease): record following stats.
- data size
- index size
- key size (uncompressed)
- value size (uncompressed)
- number of entries
- number of data blocks