From e6918187568dbd01842d8d1d2c808ce16a894239 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sun, 21 Apr 2024 13:54:28 +0200 Subject: Adding upstream version 18.2.2. Signed-off-by: Daniel Baumann --- .../_posts/2019-03-08-format-version-4.markdown | 36 ++++++++++++++++++++++ 1 file changed, 36 insertions(+) create mode 100644 src/rocksdb/docs/_posts/2019-03-08-format-version-4.markdown (limited to 'src/rocksdb/docs/_posts/2019-03-08-format-version-4.markdown') diff --git a/src/rocksdb/docs/_posts/2019-03-08-format-version-4.markdown b/src/rocksdb/docs/_posts/2019-03-08-format-version-4.markdown new file mode 100644 index 000000000..ce657696c --- /dev/null +++ b/src/rocksdb/docs/_posts/2019-03-08-format-version-4.markdown @@ -0,0 +1,36 @@ +--- +title: format_version 4 +layout: post +author: maysamyabandeh +category: blog +--- + +The data blocks in RocksDB consist of a sequence of key/values pairs sorted by key, where the pairs are grouped into _restart intervals_ specified by `block_restart_interval`. Up to RocksDB version 5.14, where the latest and default value of `BlockBasedTableOptions::format_version` is 2, the format of index and data blocks are the same: index blocks use the same key format of <`user_key`,`seq`> and encode pointers to data blocks, <`offset`,`size`>, to a byte string and use them as values. The only difference is that the index blocks use `index_block_restart_interval` for the size of _restart intervals_. `format_version=`3,4 offer more optimized, backward-compatible, yet forward-incompatible format for index blocks. + +### Pros + +Using `format_version`=4 significantly reduces the index block size, in some cases around 4-5x. This frees more space in block cache, which would result in higher hit rate for data and filter blocks, or offer the same performance with a smaller block cache size. + +### Cons + +Being _forward-incompatible_ means that if you enable `format_version=`4 you cannot downgrade to a RocksDB version lower than 5.16. + +### How to use it? + +- `BlockBasedTableOptions::format_version` = 4 +- `BlockBasedTableOptions::index_block_restart_interval` = 16 + +### What is format_version 3? +(Since RocksDB 5.15) In most cases, the sequence number `seq` is not necessary for keys in the index blocks. In such cases, `format_version`=3 skips encoding the sequence number and sets `index_key_is_user_key` in TableProperties, which is used by the reader to know how to decode the index block. + +### What is format_version 4? +(Since RocksDB 5.16) Changes the format of index blocks by delta encoding the index values, which are the block handles. This saves the encoding of `BlockHandle::offset` of the non-head index entries in each restart interval. If used, `TableProperties::index_value_is_delta_encoded` is set, which is used by the reader to know how to decode the index block. The format of each key is (shared_size, non_shared_size, shared, non_shared). The format of each value, i.e., block handle, is (offset, size) whenever the shared_size is 0, which included the first entry in each restart point. Otherwise the format is delta-size = block handle size - size of last block handle. + +The index format in `format_version=4` would be as follows: + + restart_point 0: k, v (off, sz), k, v (delta-sz), ..., k, v (delta-sz) + restart_point 1: k, v (off, sz), k, v (delta-sz), ..., k, v (delta-sz) + ... + restart_point n-1: k, v (off, sz), k, v (delta-sz), ..., k, v (delta-sz) + where, k is key, v is value, and its encoding is in parenthesis. + -- cgit v1.2.3