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Storage modes

Memgraph supports two different storage modes:

  • IN_MEMORY_TRANSACTIONAL - the default database mode that favors strongly-consistent ACID transactions using WAL files and snapshots, but requires more time and resources during data import and analysis.
  • IN_MEMORY_ANALYTICAL - speeds up import and data analysis but offers no ACID guarantees besides manually created snapshots.

You can switch between modes within the session using the following query:

STORAGE MODE IN_MEMORY_{TRANSACTIONAL|ANALYTICAL};

When switching modes, Memgraph will wait until all other transactions are done. If some other transactions are running in your system, you will receive a warning message, so be sure to set the log level to WARNING.

If you are running the Memgraph Enterprise Edition, to change the storage mode you to have STORAGE_MODE permission.

Transactional storage mode (default)​

IN_MEMORY_TRANSACTIONAL storage mode offers all ACID guarantees. WAL files and periodic snapshots are created automatically, and you can also create snapshots manually.

In the IN_MEMORY_TRANSACTIONAL mode, Memgraph creates a Delta object each time data is changed. Deltas are the backbone upon which Memgraph provides atomicity, consistency, isolation, and durability - ACID. By using Deltas, Memgraph creates write-ahead-logs for durability, provides isolation, consistency, and atomicity (by ensuring that everything is executed or nothing).

Depending on the transaction isolation level, other transactions may see changes from other transactions.

In the transactional storage mode, snapshots are created periodically or manually. They capture the database state and store it on the disk. A snapshot is used to recover the database upon startup (depending on the setting of the configuration flag --storage-recover-on-startup, which defaults to true).

When Memgraph starts creating a periodic snapshot, it is not possible to manually create a snapshot, until the periodic snapshot is created.

Manual snapshots are created by running the CREATE SNAPSHOT; query.

Analytical storage mode​

In the transactional storage mode, Memgraph is fully ACID complient which could cause memory spikes during data import because each time data is changed Memgraph creates Delta objects to provides atomicity, consistency, isolation, and durability

But Deltas also require a lot of memory (104B per change), especially when there are a lot of changes (for example, during import with the LOAD CSV clause). By switching the storage mode to IN_MEMORY_ANALYTICAL mode disables the creation of Deltas thus drastically speeding up import with lower memory consumption - up to 6 times faster import with 6 times less memory consumption.

If you want to enable ACID compliance, you can switch back to IN_MEMORY_TRANSACTIONAL and continue with regular work on the database or you can take advantage of the low memory costs of the analytical mode to run analytical queries that will not change the data, but be aware that no backup is created automatically (you can create manual snapshots), and there are no ACID guarantees besides manually created snapshots. There are no WAL files created nor periodic snapshots. Users can create a snapshot manually.

Transactions​

In the analytical storage mode, there are no ACID guarantees and other transactions can see the changes of ongoing transactions. Also, a transaction can see the changes it is doing. This means that the transactions can be committed in random orders, and the updates to the data, in the end, might not be correct.

WAL​

As mentioned, no write-ahead logs are created in the IN_MEMORY_ANALYTICAL mode. When switching back to the IN_MEMORY_TRANSACTIONAL mode it is recommended to create a snapshot manually with CREATE SNAPSHOT; Cypher query. Once Memgraph switches to the IN_MEMORY_TRANSACTIONAL mode, for all new updates it will create a WAL if not otherwise instructed by the config file.

Snapshots​

Snapshots capture the database state and store it on the disk. A snapshot is used to recover the database upon startup (depending on the setting of the configuration flag --storage-recover-on-startup, which defaults to true).

In Memgraph, snapshots are created periodically or manually.

In the IN_MEMORY_ANALYTICAL mode, periodic snapshots are disabled.

Manual snapshots are created by running the CREATE SNAPSHOT; query. When the query is run in the IN_MEMORY_ANALYTICAL mode, Memgraph guarantees that it will be the only transaction present in the system, and all the other transactions will wait until the snapshot is created to ensure its validity.