Understanding the Problem of Immediate Blocking After Failover in SQL Server

In this article, we will delve into the issue of immediate blocking occurring after a failover in a SQL Server failover cluster. We will explore the reasons behind this behavior and discuss possible solutions to mitigate or prevent it.

Background on SQL Server Failover Clusters

A SQL Server failover cluster is a high availability configuration that allows multiple servers to share resources, ensuring that no single point of failure exists. When a node in the cluster fails, the remaining nodes automatically take over its responsibilities. This process, called failover, aims to minimize downtime and ensure business continuity.

The Problem: Immediate Blocking After Failover

When a failover occurs in an SQL Server failover cluster, some tables, including our log_table, may experience immediate blocking issues. In this scenario, the table is blocked for 30-50 minutes before it can be accessed again. This phenomenon has been reported by several users and is still not well understood.

The Role of Stored Procedures

To understand why immediate blocking occurs after failover, we need to examine the stored procedures involved in our scenario: insert_sp and update_sp. These procedures are responsible for inserting and updating log entries in the log_table, respectively.

Code Analysis: Insert Sp and Update Sp

The insert_sp procedure:

ALTER PROCEDURE [dbo].[insert_sp]
    @ID varchar(32)
    @Resp varchar(max),
    @RequestDate datetime,
    @RespDate datetime,
    @ServTime float=null,
    @TranCode varchar(64) = NULL
AS
BEGIN
    INSERT INTO [dbo].[log_table] ([ID], [Resp], [RequestDate],[RespDate], [ServTime], TranCode)
     VALUES (@ID, @Resp, @RequestDate, @RespDate, @ServTime, @TranCode)
END

The update_sp procedure:

ALTER PROCEDURE [dbo].[update_sp]
    @ID varchar(32),
    @Resp varchar(max),
    @RespDate datetime,
    @ServTime float = null
AS
BEGIN
    UPDATE [dbo].[log_table]
    SET Resp = @Resp,
        RespDate = @RespDate,
        ServTime = DATEDIFF(MILLISECOND,RequestDate,@RespDate)
    WHERE
        ID = @ID AND
        RecordTime > DATEADD(HOUR, -1 , GETDATE())  
END

Both procedures use the UPDATE statement to modify the log_table. The main difference lies in what they update: new log entries (insertion) or existing ones.

Possible Causes of Immediate Blocking

Several factors might contribute to immediate blocking after failover:

• Locks: Locks are an essential part of SQL Server’s concurrency control mechanism. They prevent concurrent modifications to the same data until a lock is released. The procedure that holds the lock for the longest time will hold it longer, causing the block.
• Tran codes: Tran codes are used by SQL Server to manage transactions. When a transaction commits or rolls back, SQL Server updates the log table with the transaction details.
• RecordTime constraint: In the update_sp procedure, we can see that there is an additional condition for the update:

  WHERE
      ID = @ID AND
      RecordTime > DATEADD(HOUR, -1 , GETDATE())  
  

    This suggests that a record time is associated with each log entry. When updating existing records, we check if the `RecordTime` is within one hour of the current date.
*   **Resource availability**: SQL Server's resource allocation and scheduling mechanism might play a role in blocking when resources become available.

### Analysis and Troubleshooting

To diagnose and resolve this issue:

1.  Analyze the stored procedure execution plan to identify any performance bottlenecks or potential sources of contention.
2.  Examine lock statistics (e.g., `syslockinfo`) to see if specific locks hold for extended periods, contributing to blocking issues.
3.  Investigate Tran code activity and log table updates around failover times to determine whether concurrent transactions or data modifications are the primary cause.

## Additional Considerations

In addition to immediate blocking after failover, other factors could be worth examining:

*   **Resource configuration**: Ensure that SQL Server has sufficient resources (CPU, memory) to handle increased workloads during and after failovers.
*   **Maintenance windows**: Schedule maintenance windows strategically to minimize impact on business operations.
*   **Monitoring and alerts**: Set up monitoring tools and alert thresholds for performance metrics related to log table activity during failover periods.

## Preventing Blocking

Several strategies can be employed to mitigate or prevent blocking:

1.  Optimize query execution plans using indexing, partitioning, or recompiling procedures.
2.  Implement row-level locking (RLL) or snapshot isolation to manage concurrency and reduce contention.
3.  Schedule batch processing for log table updates during maintenance windows or less critical periods.

By understanding the root causes of immediate blocking after failover and implementing effective strategies to mitigate these issues, you can ensure the reliability and performance of your SQL Server failover cluster.

Last modified on 2024-03-11