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