DB2 Locking, Part 8: LOBs and Locking

When a row is read or modified in a table containing LOB columns, the application will obtain a normal transaction lock on the base table. The actual values for (most) LOBs are stored in a separate table space from the rest of the table data. The locks on the base table also control concurrency for the LOB table space. But DB2 uses locking strategies for large objects, too. A lock that is held on a LOB value is referred to as a LOB lock. LOB locks...

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DB2 Locking Part 7: Lock Avoidance, Related Issues, and Stuff

In today's blog entry, part 7 in our on-going series on DB2 locking, we will take a look at lock avoidance... as well as some other related things.Lock AvoidanceLock avoidance is a mechanism employed by DB2 for z/OS to access data without locking but also while maintaining data integrity. It prohibits access to uncommitted data and serializes access to pages. Lock avoidance improves performance by reducing the overall volume of lock requests. Let’s face it, the most efficient lock is the one never taken.In general, DB2 avoids locking data pages...

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DB2 Locking, Part 6: Claims, Drains, and Partition Independence

DB2 augments resource serialization using claims and drains in addition to transaction locking. The claim and drain process enables DB2 to perform concurrent operations on multiple partitions of the same table space.Claims and drains provide another “locking” mechanism to control concurrency for resources between SQL statements, utilities, and commands. But do not confuse the issue: DB2 continues to use transaction locking, as well as claims and...

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DB2 Locking, Part 5: Lock Suspensions, Timeouts, and Deadlocks

The longer a lock is held, the greater the potential impact to other applications. When an application requests a lock that is already held by another process, and the lock cannot be shared, that application is suspended. A suspended process temporarily stops running until the lock can be acquired. Lock suspensions can be a significant barrier to acceptable performance and application availability.When an application has been suspended for a pre-determined...

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