DB2 Lock Escalation and Avoidance Strategies
By Tom Nonmacher
In the world of Database Management Systems (DBMS), lock escalation is a concept that causes a significant impact on performance. Today, we will focus on DB2 11.5, a product of IBM, discussing its lock escalation mechanism and how to avoid this process. To give some context, lock escalation is a procedure where the DBMS automatically promotes row or page level locks to table level locks when a certain threshold is met. This is designed to reduce system resource consumption.
DB2 specifically escalates locks when the number of locks for a particular application approaches the maximum limit. This limit can be set using the MAXLOCKS parameter. While this is an automatic process in DB2, it can be controlled to an extent using the LOCKLIST parameter. Here is an example of how to set these parameters:
UPDATE DATABASE CONFIGURATION FOR sample
SET MAXLOCKS 20
SET LOCKLIST 2000
By increasing the LOCKLIST parameter, you can delay or avoid lock escalation. However, increasing this too much can consume significant amounts of system memory, so careful tuning is necessary. It's also worth noting that lock escalation isn't always negative—sometimes, it can improve efficiency when there are many locks at a lower level.
Another way to manage lock escalation is by tuning your queries. Minimizing the number of rows a query touches can reduce the number of locks and thus reduce the chance of escalation. Using the right isolation level can also help. For instance, the 'Cursor Stability' isolation level in DB2 holds locks for the duration of a single fetch, whereas 'Repeatable Read' holds locks until the transaction completes.
The equivalent of DB2's Repeatable Read in SQL Server 2019 and Azure SQL is SERIALIZABLE. In MySQL 8.0, it's called SERIALIZABLE as well. Here's how to set it in T-SQL (SQL Server and Azure SQL):
SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
And in MySQL:
SET SESSION TRANSACTION ISOLATION LEVEL SERIALIZABLE
Avoiding lock escalation in DB2 and other RDBMS involves a balanced mix of system configuration and query optimization. It's also important to remember that while lock escalation can cause blocking and performance issues, it's not inherently a bad thing. It's a built-in mechanism designed to manage system resources, and understanding how it works can help you better optimize your databases.
In conclusion, it’s essential to understand how DB2 handles lock escalation and how to use techniques such as adjusting parameters and optimizing queries to manage this process effectively. These principles can also be applied to other database systems such as SQL Server 2019, MySQL 8.0, Azure SQL, and Azure Synapse, helping you to design and manage your databases effectively across a range of platforms.