SQL DBA AlwaysOn scenario based interview questions – 3
One of the best known feature introduced in SQL Server 2012 is “AlwaysOn” which makes use of existing HA/DR features and provide additional features like Availability Groups. This article is for SQL Server DBA’s who are preparing for interviews, this include basic and advanced level SQL DBA AlwaysOn scenario based interview questions and answers One can easily expect scenario based questions in today’s technical interviews. Here are the scenario based questions.
SQL DBA AlwaysOn scenario based interview questions – 3:
Q. We have got an alert “WSFC cluster service is offline”. What is your action plan?
This alert is raised when the WSFC cluster is offline or in the forced quorum state. All availability groups hosted within this cluster are offline (a disaster recovery action is required).
This issue can be caused by a cluster service issue or by the loss of the quorum in the cluster.
Use the Cluster Administrator tool to perform the forced quorum or disaster recovery workflow. Once WFSC is started you must re-evaluate and reconfigure NodeWeight values to correctly construct a new quorum before bringing other nodes back online. Otherwise, the cluster may go back offline again.
Reestablishment may require if there are any High Availability features (Alwayson Availability Groups, Log Shipping, Database Mirroring) using on effected nodes.
Q. How to force a WSFC (Windows Server Failover Cluster) Cluster to start without a quorum?
This can be done using
- Failover Cluster Manager
Here we’ll see how this can be done using FCM.
Failover Cluster Manager
- Open a Failover Cluster Manager and connect to the desired cluster node to force online.
- In the Actions pane, click Force Cluster Start, and then click Yes – Force my cluster to start.
- In the left pane, in the Failover Cluster Manager tree, click the cluster name.
- In the summary pane, confirm that the current Quorum Configuration value is: Warning: Cluster is running in ForceQuorum state.
Q. We have got an alert “Availability group is offline”. Can you explain about this warning and your action plan?
This alert is raised when the cluster resource of the availability group is offline or the availability group does not have a primary replica.
- The availability group is not configured with automatic failover mode. The primary replica becomes unavailable and the role of all replicas in the availability group become RESOLVING.
- The availability group is configured with automatic failover mode and does not complete successfully.
- The availability group resource in the cluster becomes offline.
- There is an automatic, manual, or forced failover in progress for the availability group.
- If the SQL Server instance of the primary replica is down, restart the server and then verify that the availability group recovers to a healthy state.
- If the automatic failover appears to have failed, verify that the databases on the replica are synchronized with the previously known primary replica, and then failover to the primary replica. If the databases are not synchronized, select a replica with a minimum loss of data, and then recover to failover mode.
- If the resource in the cluster is offline while the instances of SQL Server appear to be healthy, use Failover Cluster Manager to check the cluster health or other cluster issues on the server. You can also use the Failover Cluster Manager to attempt to turn the availability group resource online.
- If there is a failover in progress, wait for the failover to complete.
Q. We have got an alert “Availability group is not ready for automatic failover”. Can you explain about this warning and your action plan?
This alert is raised when the failover mode of the primary replica is automatic; however none of the secondary replicas in the availability group are failover ready.
The primary replica is configured for automatic failover; however, the secondary replica is not ready for automatic failover as it might be unavailable or its data synchronization state is currently not SYNCHRONIZED.
- Verify that at least one secondary replica is configured as automatic failover. If there is not a secondary replica configured as automatic failover, update the configuration of a secondary replica to be the automatic failover target with synchronous commit.
- Use the policy to verify that the data is in a synchronization state and the automatic failover target is SYNCHRONIZED, and then resolve the issue at the availability replica.
Q. In your environment data inserted on Primary replica but not able to see that on secondary replica. When you check that Availability is in healthy state and in most cases data reflects in a few minutes but in this case it’s didn’t happen. Now you need to check for the bottleneck and fix the issue. Can you explain your views and workaround in this situation?
- Long-Running Active Transactions
- High Network Latency or Low Network Throughput Causes Log Build-up on the Primary Replica
- Another Reporting Workload Blocks the Redo Thread from Running
- Redo Thread Falls behind Due to Resource Contention
- Use DBCC OPENTRAN and check if there are any oldest transactions running on primary replica and see if they can be rolled back.
- A high DMV (sys.dm_hadr_database_replica_states) value log_send_queue_size can indicate logs being held back at the primary replica. Dividing this value by log_send_rate can give you a rough estimate on how soon data can be caught up on the secondary replica.
- Check two performance objects SQL Server:Availability Replica > Flow Control Time (ms/sec) and SQL Server:Availability Replica > Flow control/sec. Multiplying these two values shows you in the last second how much time was spent waiting for flow control to clear. The longer the flow control wait time, the lower the send rate.
- When the redo thread is blocked, an extended event called sqlserver.lock_redo_blocked is generated. Additionally, you can query the DMV sys.dm_exec_request on the secondary replica to find out which session is blocking the REDO thread, and then you can take corrective action. You can let the reporting workload to finish, at which point the redo thread is unblocked. You can unblock the redo thread immediately by executing the KILL command on the blocking session ID. The following query returns the session ID of the reporting workload that is blocking the redo thread.
Select session_id, command, blocking_session_id, wait_time, wait_type, wait_resource
where command = ‘DB STARTUP’
- When Redo Thread Falls Behind Due to Resource Contention; a large reporting workload on the secondary replica has slowed down the performance of the secondary replica, and the redo thread has fallen behind. You can use the following DMV query to see how far the redo thread has fallen behind, by measuring the difference between the gap between last_redone_lsn and last_received_lsn.
Select recovery_lsn, truncation_lsn, last_hardened_lsn,
last_received_lsn, last_redone_lsn, last_redone_time
If you see thread is indeed failing behind, do a proper investigation and take the help of resource governor and can control the CPU cycles
Note: Have a look at MSDN sites and try to understand these solutions because when you say possible solutions, immediately you might be asked about resolutions.
Q. You perform a forced manual failover on an availability group to an asynchronous-commit secondary replica, you find that data loss is more than your recovery point objective (RPO). Or, when you calculate the potential data loss of an asynchronous-commit secondary replica using the method in Monitor Performance for AlwaysOn Availability Groups, you find that it exceeds your RPO. What are the possible reasons that causes data loss is more than your recovery point objective?
There are mainly two reasons:
- High Network Latency or Low Network Throughput Causes Log Build-up on the Primary Replica. The primary replica activates flow control on the log send when it has exceeded the maximum allowable number of unacknowledged messages sent over to the secondary replica. Until some of these messages have been acknowledged, no more log blocks can be sent to the secondary replica. Since data loss can be prevented only when they have been hardened on the secondary replica, the build-up of unsent log messages increases potential data loss.
- Disk I/O Bottleneck Slows Down Log Hardening on the Secondary Replica. If the log file and the data file are both mapped to the same hard disk, reporting workload with intensive reads on the data file will consume the same I/O resources needed by the log hardening operation. Slow log hardening can translate to slow acknowledgement to the primary replica, which can cause excessive activation of the flow control and long flow control wait times.
Q. After an automatic failover or a planned manual failover without data loss on an availability group, you find that the failover time exceeds your recovery time objective (RTO). Or, when you estimate the failover time of a synchronous-commit secondary replica (such as an automatic failover partner) using the method in Monitor Performance for AlwaysOn Availability Groups, you find that it exceeds your RTO. Can you explain what are the possible reasons which causes the failover time exceeds your RTO?
- Reporting Workload Blocks the Redo Thread from Running: On the secondary replica, the read-only queries acquire schema stability (Sch-S) locks. These Sch-S locks can block the redo thread from acquiring schema modification (Sch-M) locks to make any DDL changes. A blocked redo thread cannot apply log records until it is unblocked. Once unblocked, it can continue to catch up to the end of log and allow the subsequent undo and failover process to proceed.
- Redo Thread Falls Behind Due to Resource Contention: When applying log records on the secondary replica, the redo thread reads the log records from the log disk, and then for each log record it accesses the data pages to apply the log record. The page access can be I/O bound (accessing the physical disk) if the page is not already in the buffer pool. If there is I/O bound reporting workload, the reporting workload competes for I/O resources with the redo thread and can slow down the redo thread.
Q. Let’s say you have configured Automatic failover on SQL server 2012 AlwaysOn environment. An automatic failover triggered but unsuccessful in making secondary replica as PRIMARY. How do you identify that failover is not successful and what are the possible reasons that causes an unsuccessful failover?
If an automatic failover event is not successful, the secondary replica does not successfully transition to the primary role. Therefore, the availability replica will report that this replica is in Resolving status. Additionally, the availability databases report that they are in Not Synchronizing status, and applications cannot access these databases.
Possible Reasons for Unsuccessful Failover:
- “Maximum Failures in the Specified Period” value is exhausted: The availability group has Windows cluster resource properties, such as the Maximum Failures in the Specified Period property. This property is used to avoid the indefinite movement of a clustered resource when multiple node failures occur.
- Insufficient NT Authority\SYSTEM account permissions: The SQL Server Database Engine resource DLL connects to the instance of SQL Server that is hosting the primary replica by using ODBC in order to monitor health. The logon credentials that are used for this connection are the local SQL Server NT AUTHORITY\SYSTEM login account. By default, this local login account is granted the following permissions: 1.Alter Any Availability Group, 2.Connect SQL, 3.View server state. If the NT AUTHORITY\SYSTEM login account lacks any of these permissions on the automatic failover partner (the secondary replica), then SQL Server cannot start health detection when an automatic failover occurs. Therefore, the secondary replica cannot transition to the primary role. To investigate and diagnose whether this is the cause, review the Windows cluster log.
- The availability databases are not in a SYNCHRONIZED state: In order to automatically fail over, all availability databases that are defined in the availability group must be in a SYNCHRONIZED state between the primary replica and the secondary replica. When an automatic failover occurs, this synchronization condition must be met in order to make sure that there is no data loss. Therefore, if one availability database in the availability group in the synchronizing or not synchronized state, automatic failover will not successfully transition the secondary replica into the primary role.
Q. Have you ever seen the Error 41009?
Yes! This error might occur when you try to create multiple availability groups in a SQL Server 2012 AlwaysOn failover clustering environment. This issue can be resolved by applying Cumulative Update Package 2.
Q. Let’s say you added a new file to a database which is a part of AlwaysOn Availability Groups. The add file operation succeeded on primary replica but failed in secondary replica. What is the impact and how you troubleshoot?
This might happens due to a different file path between the systems that hosts primary and secondary replica. Failed add-file operation will cause the secondary database to be suspended. This, in turn, causes the secondary replica to enter the NOT SYNCHRONIZING state.
- Remove the secondary database from the availability group.
- On the existing secondary database, restore a full backup of the filegroup that contains the added file to the secondary database, using WITH NORECOVERY and WITH MOVE (Specify the correct file path as per secondary).
- Back up the transaction log that contains the add-file operation on the primary database, and manually restore the log backup on the secondary database using WITH NORECOVERY and WITH MOVE. Restore the last transaction log file with NO RECOVERY.
- Rejoin the secondary database to the availability group.
Q. Can you write T-SQL statement for joining a replica to availability group? (AG name “ProAG”
Connect to the server instance that hosts the secondary replica and issue the below statement:
ALTER AVAILABILITY GROUP ProAG JOIN;
The same operation can be done using SSMS or using Power Shell
Q. Data synchronization state for one of the availability database is not healthy. Can you tell me the possible reasons?
If this is an asynchronous-commit availability replica, all availability databases should be in the SYNCHRONIZING state. If this is a synchronous-commit availability replica, all availability databases should be in the SYNCHRONIZED state. This issue can be caused by the following:
- The availability replica might be disconnected.
- The data movement might be suspended.
- The database might not be accessible.
- There might be a temporary delay issue due to network latency or the load on the primary or secondary replica.
Q. Let’s say we have a premium production server and it is in AlwaysOn Availability Group. You oberve that CPU utilization is hitting top at a specific time in a day. You did an RCA and found that CPU utilization reaches top and most CPU is from backup process due to backup compression is on. Now what do you suggest? Do we have any features for backup
Yes! There is an option to perform backup from secondary replicas. We can set this from Availability Group properties we can find “Backup Preferences” and from that we can choose one of the option from:
Preferred Secondary: Backups performed on Secondary if there is no secondary configured performed from primary
Secondary Only: Backups should be done from secondary only
Primary: Must occur on Primary Replica
Any Replica: Can occur from any replica in Availability Group
Q.Is there any specific limitations if we need to perform auto backups from secondary backups?
Yes! There are few:
- Only Copy_Only backup allowd from secondary replica
- Differential backups not allowed from secondary replica.
- Log backups can be performed from different secondary replicas but all these backups maintains a single log chain (LSN sequence). It might help in some of the situations
Q. Have you ever applied patches / CU / service packs on Alwayson Availability Groups? Did you face any issues while applying?
Yes! I have applied CU and service packs on SQL Server 2012 SP2 Cumulative Update 4
I had a bad experience with Alwayson AG:
After CU4 applied we saw that AlwaysOn vailiabilty Gropus are in Non- Synchronizing state.
After RCA we found that there was a huge blocking between user sessions and a unknown session, CHECKPOINT with command running as “DB_STARTUP”.
Through of the MSDN SITE we found that Microsoft declared it’s a bug and the solution chosen as below:
- We had to open an outage:
- Disable Automatic Failover
- Restart the SQL Server on Primary Replica
- Re-enable automatic failover.
- This worked and fixed the issue.
Q. Can you explain any difficult issue you have faced recently on High Availability Groups?
Sure! We are configuring AlwaysOn AG on SQL server 2014.
We have taken backup from Primary replica and restored on secondary replica
When we are trying to add secondary replica to availability group to our surprise sql server got shut down and we found the error message:
(Error: 3449, Severity: 21, State: 1.
SQL Server must shut down in order to recover a database (database ID 1). The database is either a user database that could not be shut down or a system database. Restart SQL Server. If the database fails to recover after another startup, repair or restore. SQL Trace was stopped due to server shutdown. Trace ID = ‘1’. This is an informational message only; no user action is required. )
We did RCA and found the below.
- Service broker is enabled at Primary Replica
- We have taken a full backup from Primary Replica
- Restored on Secondary Replica where Service Broker is not enabled
- When we try to add secondary replica to AG, Service Broker is enabled, the same GUID on availability database is detected which causes an silent error 9772:
- “The Service Broker in database “<dbname>” cannot be enabled because there is already an enabled Service Broker with the same ID”.
- This results into error 3449 and shut down the sql server unexpectedly.
This has been fixed by applying the CU1 on SQL Server 2014.
Q. Replica is in “resolving” status? What does it mean?
A replica is into “RESOLVING” state when a auto failover is not successful.
Additionally the availability databases reports that they are in non-synchronizing state and not accessible.
Q. What are the top reasons that cause an unsuccessful failover?
- Auto failovers in a specific period may crossed the value “Maximum Failures in the Specified Period”
- Insufficient NT Authority\SYSTEM account permissions
- The availability databases are not in a SYNCHRONIZED state
We should be thankful for the authors who has given an excellent explanations on AlwaysOn topics from SQL Server 2012 and SQL Server 2014. Here is the list of top references: