leveldb 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 varints for an explanation of varint64 format.
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.
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.ccand then optionally compressed.
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.
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.
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
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
<N> is the string returned by the filter policy's
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
[ 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
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