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What is the difference between sys.dm_hadr_ tables and sys.availability_groups / sys.availability_replicas?

Database Administrators Asked on January 6, 2022

Browsing around some sys tables, I see:

sys.dm_hadr_availability_replica_cluster_nodes.group_name = sys.availability_groups.name

and

sys.dm_hadr_availability_replica_cluster_nodes.replica_server_name = sys.availability_replicas.replica_server_name

and

sys.dm_hadr_availability_replica_cluster_nodes.node_name = sys.availability_replicas.replica_server_name

and

sys.dm_hadr_database_replica_states.group_id = sys.availability_groups.group_id

Could someone please clarify for me whether the “hadr” tables are duplicating data or not? For example, sys.dm_hadr_availability_replica_nodes and sys.avilability_replicas sound like they’re describing the same thing.

I guess the question is on how safe it is to join these tables given I can see data values equate – because after all, the same value (something like ‘SQL01’) could conceivably be used to describe different things.

Better yet – is there no entity-relationship diagram published by Microsoft for these sys tables? I can find old posts where people ask the same question about ERDs and there are answers regarding older versions of SQL Server, but all links to diagrams seem outdated and defunct.

One Answer

It's not duplicating of the information, depending on the requirement we could utilize anyone them:

  1. DMV: The prefix sys.dm_ represents DMV the under lying information from Dynamic Management views
  2. Catalog View: No prefix sys. represents catalog views. For more details..

Answered by Shekar Kola on January 6, 2022

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