There are 4 log replication modes:
Synchronous in-memory (syncmem): This is the default log replication mode. In this mode, primary node waits for the acknowledgement confirming the log has been received by the secondary node before committing any transactions.
Basically, primary node waits until secondary node has received data and as long as the replication status is ACTIVE for all services, there will not be any data loss.
The waiting period of the primary system is determined by parameter named logshipping_timeout in global.ini. Its default value is 30 secs and if there is no acknowledgement received from the secondary node after 30 secs, primary node continues without replicating the data.
Obviously, this option provides the best performance among all four options as the primary node does not have to wait for any data transfer between nodes, or I/O activity on the secondary node. However, async mode is more vulnerable to data loss compared to other options. You might expect some data loss during an (especially unexpected) failovers.
Synchronous in-memory (syncmem): This is the default log replication mode. In this mode, primary node waits for the acknowledgement confirming the log has been received by the secondary node before committing any transactions.
Basically, primary node waits until secondary node has received data and as long as the replication status is ACTIVE for all services, there will not be any data loss.
The waiting period of the primary system is determined by parameter named logshipping_timeout in global.ini. Its default value is 30 secs and if there is no acknowledgement received from the secondary node after 30 secs, primary node continues without replicating the data.
Synchronous (sync): In this mode, primary node waits for acknowledgement confirming the log has been received AND persisted by the secondary node before committing any transactions.
The key benefit of this option compared to syncmem is the consistency between primary and secondary nodes. You know that the primary node will not commit any transactions until secondary node received and persisted the logs.
Like syncmem, the waiting period of the primary system is also 30 secs by default and determined by logshipping_timeout parameter in global.ini. If there is no acknowledgement received from the secondary node after 30 secs, primary node continues without replicating the data.
Synchronous (full sync): Full sync replication was introduced with SPS08 as an additional option for the sync mode. This mode provides absolute zero data loss no matter what because primary node waits until secondary node received the logs and persisted them on the disk; the transaction processing on the primary node is blocked until secondary system becomes available. This ensures no transaction can be committed on the primary node without shipping the logs to the secondary site.
Asynchronous (async): In this option, primary node does not wait any acknowledgement or confirmation from the secondary node, it commits the transactions when it has been written to the log file of the primary system and sent redo logs to the secondary node asynchronously. Obviously, this option provides the best performance among all four options as the primary node does not have to wait for any data transfer between nodes, or I/O activity on the secondary node. However, async mode is more vulnerable to data loss compared to other options. You might expect some data loss during an (especially unexpected) failovers.
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