The journal block write synchronization mode (the default synchronization mode) writes journal blocks across a network connection to secondary databases as they are written to the primary database journal. In the journal block write mode, SDS can establish and maintain very close synchronization between primary and secondary databases.
When the secondary database is synchronized with the primary, audit blocks are transferred and written to the secondary database journal at the same time they are written to the journal on the primary. In effect, primary journal writes are mirrored to the secondary journal. These mirrored journal writes are performed asynchronously so that primary database transaction processing is not held up waiting for acknowledgements from secondary databases.
The principal benefits of using the journal block write synchronization mode are as follows.
Synchronization of journals on the primary and secondary databases is closer than is possible with the journal switch mode.
Loss of audit data is minimized and a disaster recovery therefore recovers most, if not all, transactions that committed on the primary database.
Following a takeover, restoration of database access can be faster than with the journal switch mode.
A block write secondary is caught up to the primary when it is actively replaying in the current primary write journal.