System Parameters

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	DB2 Universal Database for OS/390 v7.1 Application Certification Guide By Susan Lawson
	Table of Contents
	Chapter 16.  Locking and Concurrency

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System Parameters

Table 16-5 lists some system parameters (DSNZPARMs) that are used to control aspects of locking.

Table 16-5. System Parameters for Locking

Parameter	Description
RECURHL	The use of the RECURHL = YES DSNZPARM can help with concurrency because it allows DB2 to release the locks that are held by a cursor defined WITH HOLD.
IRLMRWT	This is the number of seconds that a transaction will wait for a lock before a timeout is detected . The IRLM uses this for timeout and deadlock detection. Most shops take the default of 60 seconds, but in some high-performance situations where detection must occur sooner (so the applications are not incurring excessive lock wait times), it is been set lower. If timeouts or deadlocks are hit often, the applications is reviewed and tuned .
XLKUPDLT	This DSNZPARM, new with Version 6, allows you to specify the locking method to use when a searched UPDATE or DELETE is performed. The default is NO, best for concurrency, which says that DB2 will use an S or U lock when scanning qualifying rows and then upgrades to an X lock when a qualifying row is found. The value of YES is useful in a data sharing environment when the searches involve an index, because it takes an X lock on qualifying rows or pages.
NUMLKTS	This represents the maximum number of locks on an object. If you are turning off lock escalation (LOCKMAX 0), you will need to increase this number. If you are using LOCKMAX SYSTEM, then the value here will be the value for SYSTEM.
NUMLKUS	This is the maximum number of page or row locks that a single application can have held concurrently on all tablespaces. This includes data pages, index pages, subpages, and rows. If you specify 0, then there is no limit on the number of locks. You want to be careful with 0, because if you turn off lock escalation and do not commit frequently enough, you could run into storage problems (DB2 uses 250 bytes for each lock).

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