IO

Control Write I/O

Range Sync

RocksDB's data files are usually generated in an appending way. File system may choose buffer the write until the dirty pages hit a threshold and write out all of those pages all together. This can create a burst of write I/O and cause the online I/Os to wait too long and cause long query latency. Rather, you can ask RocksDB to periodically hint OS to write out outstanding dirty pages by setting options.bytes_per_sync for SST files and options.wal_bytes_per_sync for WAL files. Underlying it calls sync_file_range() on Linux every time a file is appended for such size. The most recent pages are not included in the range sync.

Rate Limiter

You can control the total rate RocksDB writes to data files through options.rate_limiter, in order to reserve enough I/O bandwidth to online queries. See Rate Limiter for details.

Write Max Buffer

When appending a file, RocksDB has internal buffering of files before writing to the file system, unless an explicit fsync is needed. The max size of this buffer can be controlled by options.writable_file_max_buffer_size. Tuning this parameter is more critical in [[Direct IO] mode or to a file system without page cache. With non-direct I/O mode, enlarging this buffer only reduces number of write() system calls and is unlikely to change the I/O behavior, so unless this is what you want, it may be desirable to keep the default value 0 to save the memory.

Control Read I/O

Coming soon.

Direct I/O

Rather than control the I/O through file system hints shown above, you can enable direct I/O in RocksDB to allow RocksDB to directly control I/O, using option use_direct_reads and/or use_direct_io_for_flush_and_compaction. If direct I/O is enabled, some or all of the options introduced above will not be applicable. See more details in Direct IO.