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.