RocksDB 4.1 发布，更新内容如下：
Added single delete operation as a more efficient way to delete keys that have not been overwritten.
Added experimental AddFile() to DB interface that allow users to add
files created by SstFileWriter into an empty Database, see
include/rocksdb/sst_file_writer.h and DB::AddFile() for more info.
Added support for opening SST files with .ldb suffix which enables opening LevelDB databases.
CompactionFilter now supports filtering of merge operands and merge results.
公共 API 更新
Added SingleDelete() to the DB interface.
Added AddFile() to DB interface.
Added SstFileWriter class.
CompactionFilter has a new method FilterMergeOperand() that RocksDB
applies to every merge operand during compaction to decide whether to
filter the operand.
We removed CompactionFilterV2 interfaces from
include/rocksdb/compaction_filter.h. The functionality was deprecated
already in version 3.13.
RocksDB 是一个来自 facebook 的可嵌入式的支持持久化的 key-value 存储系统，也可作为 C/S 模式下的存储数据库，但主要目的还是嵌入式。RocksDB 基于 LevelDB 构建。关于 RocksDB 的性能说明。