MySQL Cluster is mainly an in-memory database. Nevertheless it requires a good I/O system for writing various different information to disk.

The information MySQL Cluster writes to disk are the:

• Global Checkpoints (GCP) which are the transactions.
• Local Checkpoints (LCP) which is a dirty image of the data.
• Backup.

In the following schema (a 2-node Cluster) you can see what is related to each other:

Please find here the meaning of each parameter:

The parameter DiskSyncSize is a MySQL Cluster parameter and was added in MySQL 5.1.23.

After the amount of stored bytes of data per file, the data node will fsync (flush) the LCP file to disk, even if a fsync is not needed for consistency.
This is done because the OS will otherwise buffer all the writes, and when a fsync is really needed, it can take a lot of time...

Originally this parameter was hard coded. Now it defaults to 4 Mbyte.