You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
104 lines
3.7 KiB
104 lines
3.7 KiB
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
|