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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


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

Write buffers


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.


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.

Introduced in: v3.3.20

The total amount of data to build up in all in-memory buffers (backed by log files, in bytes). This option, together with the block cache size configuration option, can be used to limit memory usage.

If set to 0, the memory usage is not limited. This is the default setting in 3.3. Please note that the default setting may be adjusted in future versions of ArangoDB.

If set to a value greater than 0, this will cap the memory usage for write buffers, but may have an effect on write performance.


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


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.


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


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


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.


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.


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.


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.


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.


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.


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

File I/O


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.


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


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


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

Background tasks


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


Number of threads for high priority operations (e.g. flush). We recommend setting this equal to max-background-flushes. Default: number of processors / 2.


Number of threads for low priority operations (e.g. compaction). Default: number of processors / 2.



This is the maximum 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.


This option is ignored because it could cause errors in RocksDB 5.6. Consider upgrading to ArangoDB v3.4 or later with a more recent RocksDB bundled.

Whether or not the maximum size of the RocksDB block cache is strictly enforced. This option can be set to limit the memory usage of the block cache to at most the specified size. If then inserting a data block into the cache would exceed the cache’s capacity, the data block will not be inserted. If the flag is not set, a data block may still get inserted into the cache. It is evicted later, but the cache may temporarily grow beyond its capacity limit.


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.


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


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



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.


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

Non-Pass-Through Options


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:


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”).


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.


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


If enabled, throttles the ingest rate of writes if necessary to reduce chances of compactions getting too far behind and blocking incoming writes. This option is true by default.


The interval (in milliseconds) that ArangoDB will use to automatically synchronize data in RocksDB’s write-ahead logs to disk. Automatic syncs will only be performed for not-yet synchronized data, and only for operations that have been executed without the waitForSync attribute.

The default sync interval in 3.3 is 0, meaning that automatic background syncing is turned off. Automatic syncing was added in the middle of the ArangoDB 3.3 release cycle, so it is opt-in. The default sync interval will change to 100 milliseconds in ArangoDB 3.4 however.

Note: this option is not supported on Windows platforms. Setting the option to a value greater 0 will produce a startup warning.


When set to true, enables writing of RocksDB’s own informational LOG files into RocksDB’s database directory.

This option is turned off by default, but can be enabled for debugging RocksDB internals and performance.


When set to true, enables verbose logging of RocksDB’s actions into the logfile written by ArangoDB (if option --rocksdb.use-file-logging is off) or RocksDB’s own log (if option --rocksdb.use-file-logging is on).

This option is turned off by default, but can be enabled for debugging RocksDB internals and performance.