RocksDB engine options

RocksDB is a highly configurable key-value store used to power our RocksDB storage engine. Most of the options on this page are pass-through options to the underlying RocksDB instance, and we change very few of their default settings.

Depending on the storage engine you have chosen the availability and the scope of these options changes.

In case you have chosen mmfiles some of the following options apply to persistent indexes. In case of rocksdb it will apply to all data stored as well as indexes.

Pass-through options

--rocksdb.wal-directory

Absolute path for the RocksDB WAL files. If left empty, this will use a subdirectory journals inside the data directory.

Write buffers

--rocksdb.write-buffer-size

The amount of data to build up in each in-memory buffer (backed by a log file) before closing the buffer and queuing it to be flushed into standard storage. Default: 64MiB. Larger values may improve performance, especially for bulk loads.

--rocksdb.max-write-buffer-number

The maximum number of write buffers that built up in memory. If this number is reached before the buffers can be flushed, writes will be slowed or stalled. Default: 2.

--rocksdb.min-write-buffer-number-to-merge

Minimum number of write buffers that will be merged together when flushing to normal storage. Default: 1.

--rocksdb.max-total-wal-size

Maximum total size of WAL files that, when reached, will force a flush of all column families whose data is backed by the oldest WAL files. Setting this to a low value will trigger regular flushing of column family data from memtables, so that WAL files can be moved to the archive. Setting this to a high value will avoid regular flushing but may prevent WAL files from being moved to the archive and being removed.

--rocksdb.delayed-write-rate (Hidden)

Limited write rate to DB (in bytes per second) if we are writing to the last in-memory buffer allowed and we allow more than 3 buffers. Default: 16MiB/s.

LSM tree structure

--rocksdb.num-levels

The number of levels for the database in the LSM tree. Default: 7.

--rocksdb.num-uncompressed-levels

The number of levels that do not use compression. The default value is 2. Levels above this number will use Snappy compression to reduce the disk space requirements for storing data in these levels.

--rocksdb.dynamic-level-bytes

If true, the amount of data in each level of the LSM tree is determined dynamically so as to minimize the space amplification; otherwise, the level sizes are fixed. The dynamic sizing allows RocksDB to maintain a well-structured LSM tree regardless of total data size. Default: true.

--rocksdb.max-bytes-for-level-base

The maximum total data size in bytes in level-1 of the LSM tree. Only effective if --rocksdb.dynamic-level-bytes is false. Default: 256MiB.

--rocksdb.max-bytes-for-level-multiplier

The maximum total data size in bytes for level L of the LSM tree can be calculated as max-bytes-for-level-base * (max-bytes-for-level-multiplier ^ (L-1)). Only effective if --rocksdb.dynamic-level-bytes is false. Default: 10.

--rocksdb.level0-compaction-trigger

Compaction of level-0 to level-1 is triggered when this many files exist in level-0. Setting this to a higher number may help bulk writes at the expense of slowing down reads. Default: 2.

--rocksdb.level0-slowdown-trigger

When this many files accumulate in level-0, writes will be slowed down to --rocksdb.delayed-write-rate to allow compaction to catch up. Default: 20.

--rocksdb.level0-stop-trigger

When this many files accumulate in level-0, writes will be stopped to allow compaction to catch up. Default: 36.

File I/O

--rocksdb.compaction-read-ahead-size (Hidden)

If non-zero, we perform bigger reads when doing compaction. If you're running RocksDB on spinning disks, you should set this to at least 2MiB. That way RocksDB's compaction is doing sequential instead of random reads. Default: 0.

--rocksdb.use-direct-reads (Hidden)

Only meaningful on Linux. If set, use O_DIRECT for reading files. Default: false.

--rocksdb.use-direct-io-for-flush-and-compaction (Hidden)

Only meaningful on Linux. If set, use O_DIRECT for writing files. Default: false.

--rocksdb.use-fsync (Hidden)

If set, issue an fsync call when writing to disk (set to false to issue fdatasync only. Default: false.

Background tasks

--rocksdb.base-background-compactions (Hidden)

Suggested number of concurrent background compaction jobs, submitted to the low priority thread pool. Default: 1.

--rocksdb.max-background-compactions

Maximum number of concurrent background compaction jobs, submitted to the low priority thread pool. Default: 1.

--rocksdb.max-background-flushes

Maximum number of concurrent flush operations, submitted to the high priority thread pool. Default: 1.

--rocksdb.num-threads-priority-high

Number of threads for high priority operations (e.g. flush). We recommend setting this equal to max-background-flushes. Default: 1.

--rocksdb.num-threads-priority-low

Number of threads for low priority operations (e.g. compaction). We recommend setting this equal to max-background-compactions. Default: 1.

Caching

--rocksdb.block-cache-size

This is the size of the block cache in bytes. Increasing this may improve performance. If there is less than 4GiB of RAM on the system, the default value is 256MiB. If there is more, the default is (system RAM size - 2GiB) * 0.3.

--rocksdb.block-cache-shard-bits

The number of bits used to shard the block cache to allow concurrent operations. To keep individual shards at a reasonable size (i.e. at least 512KB), keep this value to at most block-cache-shard-bits / 512KB. Default: block-cache-size / 2^19.

--rocksdb.table-block-size

Approximate size of user data (in bytes) packed per block for uncompressed data.

--rocksdb.recycle-log-file-num (Hidden)

Number of log files to keep around for recycling. Default: 0.

Miscellaneous

--rocksdb.optimize-filters-for-hits (Hidden)

This flag specifies that the implementation should optimize the filters mainly for cases where keys are found rather than also optimize for the case where keys are not. This would be used in cases where the application knows that there are very few misses or the performance in the case of misses is not as important. Default: false.

--rocksdb.wal-recovery-skip-corrupted (Hidden)

If true, skip corrupted records in WAL recovery. Default: false.

Non-Pass-Through Options

--rocksdb.wal-file-timeout (Hidden)

Timeout after which unused WAL files are deleted (in seconds). Default: 10.0s.

Data of ongoing transactions is stored in RAM. Transactions that get too big (in terms of number of operations involved or the total size of data created or modified by the transaction) will be committed automatically. Effectively this means that big user transactions are split into multiple smaller RocksDB transactions that are committed individually. The entire user transaction will not necessarily have ACID properties in this case.

The following options can be used to control the RAM usage and automatic intermediate commits for the RocksDB engine:

--rocksdb.max-transaction-size

Transaction size limit (in bytes). Transactions store all keys and values in RAM, so large transactions run the risk of causing out-of-memory sitations. This setting allows you to ensure that does not happen by limiting the size of any individual transaction. Transactions whose operations would consume more RAM than this threshold value will abort automatically with error 32 ("resource limit exceeded").

--rocksdb.intermediate-commit-size

If the size of all operations in a transaction reaches this threshold, the transaction is committed automatically and a new transaction is started. The value is specified in bytes.

--rocksdb.intermediate-commit-count

If the number of operations in a transaction reaches this value, the transaction is committed automatically and a new transaction is started.