Write-ahead log

ArangoDB’s RocksDB storage engine stores all data-modification operation in a write-ahead log (WAL). The WAL is sequence of append-only files containing all the write operations that were executed on the server. It is used to run data recovery after a server crash, and can also be used in a replication setup when Followers need to replay the same sequence of operations as on the Leader.

The individual RocksDB WAL files are per default about 64 MiB big. The size will always be proportionally sized to the value specified via --rocksdb.write-buffer-size. The value specifies the amount of data to build up in memory (backed by the unsorted WAL on disk) before converting it to a sorted on-disk file.

Larger values can increase performance, especially during bulk loads. Up to --rocksdb.max-write-buffer-number write buffers may be held in memory at the same time, so you may wish to adjust this parameter to control memory usage. A larger write buffer will result in a longer recovery time the next time the database is opened.

The RocksDB WAL only contains committed transactions. This means you will never see partial transactions in the replication log, but it also means transactions are tracked completely in-memory. In practice this causes RocksDB transaction sizes to be limited, for more information see the RocksDB Configuration