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:

1
2
3
4
5

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.