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When does data synchronization in SQL Server always on availability?
Users will receive commit acknowledgement as soon as it gets committed in Primary replica. In the following screenshot, you can see this scenario. Once the secondary replica is available and connects with the Primary replica, it again starts Synchronous data commit.
How is lag measured in availability group synchronization?
This is an area that we need to monitor, particularly if the secondary replica is underpowered when compared to the primary replica. Dividing the redo_queue by the redo_rate will indicate your lag. Everything discussed thus far has revolved around recovery in asynchronous commit mode.
Is there an hour lag in database synchronization?
If a transaction occurs at 02:00am and then the write load on the database goes idle for one hour, this calculation will be misleading until the next transaction is synchronized. The metric would declare a one-hour lag even though there was no data to be lost during that hour. While misleading, the hour lag is technically accurate.
How to estimate the time to full synchronization?
To estimate the time to full synchronization and to identify the bottleneck, you need to understand the synchronization process. Performance bottleneck can be anywhere in the process, and locating the bottleneck can help you dig deeper into the underlying issues. The following figure and table illustrate the data synchronization process:
What does always on availability mean in SQL Server?
We can have a Primary instance and multiple secondary instances (depending upon SQL Version and Edition). Few important terms of SQL Server Always on Availability Groups are as follows. Availability group: It is a logical group of user databases that should failover together on the secondary replica
What does synchronous commit mean in availability group?
Within a given availability group, a pair of availability replicas (including the current primary replica) that are configured for synchronous-commit mode with automatic failover, if any. An automatic failover settakes effect only if the secondary replica is currently SYNCHRONIZED with the primary replica.
How does asynchronous data commit mode in SQL Server?
In Asynchronous data commit mode, the primary replica does not wait for the acknowledgement from participating secondary replica. It sends the acknowledgement to the client for transaction commit In the secondary replica, Log Receive gets the log records from the primary replica and writes to Log cache.