SQL Server:Database Replica > Redone Bytes/secĪvailability groups are designed with flow control gates on the primary replica to avoid excessive resource consumption, such as network and memory resources, on all availability replicas. Pages are kept in the redo queue as they wait to be redone. Redo the flushed pages on the secondary replica. Performance counter SQL Server:Database > Log Bytes Flushed/sec
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Once the log is hardened, data loss is avoided. After the log flush, an acknowledgment is sent back to the primary replica. Log is flushed on the secondary replica for hardening. Performance counter SQL Server:Availability Replica > Log Bytes Received/sec SQL Server:Availability Replica > Bytes sent to transport\secĮach secondary replica receives and caches the message. The messages in each database-replica queue is dequeued and sent across the wire to the respective secondary replica. Log_send_queue_size (KB) and log_bytes_send_rate (KB/sec) on the primary replica. SQL Server:Availability Replica > Bytes Sent to Replica\sec, which is an aggregation of the sum of all database messages queued for that availability replica. If the capture process is not able to scan and enqueue the messages fast enough, the log send queue builds up. This capture process runs continuously as long as the availability replica is connected and data movement is not suspended for any reason, and the database-replica pair is shown to be either Synchronizing or Synchronized. Logs for each database is captured and sent to the corresponding partner queue (one per database-replica pair). SQL Server:Database > Log bytes flushed\sec This log must be replicated to the secondary replicas.
The following figure and table illustrate the data synchronization process: Performance bottleneck can be anywhere in the process, and locating the bottleneck can help you dig deeper into the underlying issues.
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To estimate the time to full synchronization and to identify the bottleneck, you need to understand the synchronization process.
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This article describes the synchronization process, shows you how to calculate some of the key metrics, and gives you the links to some of the common performance troubleshooting scenarios. Understanding how availability groups ship logs to secondary replicas can help you estimate the recovery time objective (RTO) and recovery point objective (RPO) of your availability implementation and identify bottlenecks in poorly performing availability groups or replicas. The performance aspect of Always On Availability Groups is crucial to maintaining the service-level agreement (SLA) for your mission-critical databases. Applies to: SQL Server (all supported versions)