Measuring Availability Group Synchronization Lag

With all of the high-availability (HA) and disaster recovery (DR) features, the database administrator must understand how much data loss and downtime is possible under the worst case scenarios. Data loss affects your ability to meet recovery point objectives (RPO) and downtime affects your recovery time objectives (RTO). When using Availability Groups (AGs), your RTO and RPO rely upon the replication of transaction log records between at least two replicas to be extremely fast. The worse the performance, the more potential data loss will occur and the longer it can take for a failed over database to come back online.

Availability Groups must retain all transaction log records until they have been distributed to all secondary replicas. Slow synchronization to even a single replica will prevent log truncation. If the log records cannot be truncated your log will likely begin to grow. This becomes a maintenance concern because you either need to continue to expand your disk or you might run out of capacity entirely.

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