Both storage engines use a form of write ahead logging (WAL).
Starting with version 2.2 ArangoDB stores all data-modification operation in its write-ahead log. The write-ahead log 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 slaves need to replay the same sequence of operations as on the master.
MMFiles WAL Details
By default, each write-ahead logfile is 32 MiB in size. This size is configurable via the option --wal.logfile-size. When a write-ahead logfile is full, it is set to read-only, and following operations will be written into the next write-ahead logfile. By default, ArangoDB will reserve some spare logfiles in the background so switching logfiles should be fast. How many reserve logfiles ArangoDB will try to keep available in the background can be controlled by the configuration option --wal.reserve-logfiles.
Data contained in full write-ahead files will eventually be transferred into the journals or datafiles of collections. Only the "surviving" documents will be copied over. When all remaining operations from a write-ahead logfile have been copied over into the journals or datafiles of the collections, the write-ahead logfile can safely be removed if it is not used for replication.
Long-running transactions prevent write-ahead logfiles from being fully garbage-collected because it is unclear whether a transaction will commit or abort. Long-running transactions can thus block the garbage-collection progress and should therefore be avoided at all costs.
On a system that acts as a replication master, it is useful to keep a few of the already collected write-ahead logfiles so replication slaves still can fetch data from them if required. How many collected logfiles will be kept before they get deleted is configurable via the option --wal.historic-logfiles.
For all write-ahead log configuration options, please refer to the page Write-ahead log options.
RocksDB WAL Details
The options mentioned above only apply for MMFiles. The WAL in the rocksdb storage engine
works slightly differently.
Note: In rocksdb the WAL options are all prefixed with
--wal.* options do have no effect.
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
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