ArangoDB Server RocksDB 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.total-write-buffer-size

The total amount of data to build up in all in-memory buffers (backed by log files). 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.

If set to a value larger than 0, this will cap memory usage for write buffers but may have an effect on performance. If there is less than 4GiB of RAM on the system, the default value is 512MiB. If there is more, the default is (system RAM size - 2GiB) * 0.5.

--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

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

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

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

--rocksdb.use-direct-io-for-flush-and-compaction

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

--rocksdb.use-fsync

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

--rocksdb.allow-fallocate

Allow RocksDB to use the fallocate call. If false, fallocate calls are bypassed and no preallocation is done. Preallocation is turned on by default, but can be turned off for operating system versions that are known to have issues with it. This option only has an effect on operating systems that support fallocate.

--rocksdb.limit-open-files-at-startup

If set to true, this will limit the amount of .sst files RocksDB will inspect at startup, which can reduce the number of IO operations performed at start.

--rocksdb.block-align-data-blocks

If true, data blocks are aligned on the lesser of page size and block size, which may waste some memory but may reduce the number of cross-page I/O operations.

Background tasks

--rocksdb.max-background-jobs

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

--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: number of processors / 2.

--rocksdb.num-threads-priority-low

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

Caching

--rocksdb.block-cache-size

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.

--rocksdb.enforce-block-cache-size-limit

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.

--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

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

Miscellaneous

--rocksdb.optimize-filters-for-hits

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

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

Non-Pass-Through Options

--rocksdb.wal-file-timeout

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.wal-file-timeout-initial

Timeout after which deletion of unused WAL files kicks in after server start (in seconds). Default: 180.0s

By decreasing this option’s value, the server will start the removal of obsolete WAL files earlier after server start. This is useful in testing environments that are space-restricted and do not require keeping much WAL file data at all.

--rocksdb.wal-archive-size-limit

Maximum total size (in bytes) of archived WAL files to keep on a leader. A value of 0 will not restrict the size of the archive, so the leader will removed archived WAL files when there are no replication clients needing them. Any non-zero value will restrict the size of the WAL files archive to about the specified value and trigger WAL archive file deletion once the threshold is reached. Please note that the value is only a threshold, so the archive may get bigger than the configured value until the background thread actually deletes files from the archive. Also note that deletion from the archive will only kick in after --rocksdb.wal-file-timeout-initial seconds have elapsed after server start.

The default value is 0 (i.e. unlimited).

When setting the value to a size bigger than 0, the RocksDB storage engine will force a removal of archived WAL files if the total size of the archive exceeds the configured size. The option can be used to get rid of archived WAL files in a disk size-constrained environment.

Note that archived WAL files are normally deleted automatically after a short while when there is no follower attached that may read from the archive. However, in case when there are followers attached that may read from the archive, WAL files normally remain in the archive until their contents have been streamed to the followers. In case there are slow followers that cannot catch up this will cause a growth of the WAL files archive over time.

The option --rocksdb.wal-archive-size-limit can now be used to force a deletion of WAL files from the archive even if there are followers attached that may want to read the archive. In case the option is set and a leader deletes files from the archive that followers want to read, this will abort the replication on the followers. Followers can however restart the replication doing a resync.

--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 situations. 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.

--rocksdb.throttle

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.

--rocksdb.sync-interval

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.

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

--rocksdb.use-file-logging

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.

--rocksdb.debug-logging

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.