SAP HANA HA and DR Series #7: Log Replication Modes
Good day everyone,
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. However, if at least one service has a status different from ACTIVE, a failover to secondary node might result in a data loss. Because there might be some committed changes in the primary node which could not be sent to secondary due to a connectivity or service failure. So, if replication status is active for all services, you don’t lose any data in case of a failover.
This option offers two main advantages: shorter transaction delay and better performance. Because the primary node waits for secondary node only to “receive” the logs, the transaction delay in the primary node is shorter (which is literally the log transmission time) compared to other replication modes. Also, because the primary node does not wait for any I/O or disk writing activity on the secondary node, it basically performs better.
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.
This replication mode can be ideal for system replication setup as a high availability and disaster recovery solution, especially if both nodes are in the same data center or very close to each other.
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.
This replication mode can be ideal for system replication setup as a high availability solution, especially if both nodes are in the same data center.
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.
This mode can be activated via parameter enable_full_sync in the system replication section of global.ini file. When the parameter is set to disabled, full sync is not configured. If you change the parameter to enabled in a running system, that means full sync is configured but not activated immediately to prevent the transaction blocking.
Full sync will be completely activated only when the parameter is enabled AND the secondary node connected. Then you will see the REPLICATION_STATUS becomes ACTIVE. That means if there is a network connectivity issue between two nodes transactions on the primary node will be blocked until secondary node is back.
This replication mode can be ideal for multitier system replication configurations especially between tier 2 and tier 3 nodes for data protection, or can also be used for system replication HA configuration when both nodes are in the same local area network and data protection is number one priority.
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.
This replication mode can be ideal for system replication as a DR solution in a distant location or for companies where system performance is the number one priority and because it provides a database consistency with a near zero data loss, it is still a favourable option. In a multitier system replication, async can also be used between tier 2 and tier 3 as a DR solution.
Have any questions about SAP HANA System Replication? Leave a comment below.
References and further reading:
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