DB2 claim与drain

A claim is a notification to DB2 that an object is being accessed. When an application first accesses an object, within a unit of work, it makes a claim on the

object. It releases the claim at the next commit point. Unlike a transaction lock, a claim normally does not persist past the commit point. To access the object in the next unit of work,

the application must make a new claim.(claim的生命周期在一个unit work)

However, an exception exists: If a cursor defined with the WITH HOLD clause is positioned on the claimed object, the claim is not released at a commit point.

A claim indicates activity on or interest in a particular page set or partition to DB2. Claims prevent drains from occurring until the claim is released.

Table 6-1 shows the three classes of claims and the actions that they allow.

Table 6-1 Claim classes and how DB2 uses them

Claim class Notification to DB2 that the following action is being performed on the object involved

Write Reading, updating, inserting, and deleting

Repeatable read Reading only, with repeatable read (RR) isolation

Cursor stability read Reading only, with read stability (RS), cursor stability (CS), or uncommitted read (UR) isolation

A drain is the action of taking over access to an object by preventing new claims and waiting for existing claims to be released.

A utility can drain a partition when applications are accessing it. The drain quiesces the applications by allowing each one to reach a commit point, but preventing any of them, or any

other applications, from making a new claim. drain允许原有claim一直活到commit point，但不允许新的claim。When no more claims exist, the process that drains (the drainer) controls access to the drained object. The applications that were drained can still hold transaction locks on the drained object, but they cannot make new claims until the drainer has finished.

A drainer does not always need complete control. It could drain the following combinations of

claim classes:

Only the write claim class

Only the repeatable read claim class

All claim classes

For example, the CHECK INDEX utility needs to drain only writers from an index space and its associated table space. The RECOVER TABLESPACE utility, however, must drain all claim

classes from its table space. The REORG utility can drain either writers (with DRAIN WRITERS) or all claim classes (with DRAIN ALL).

A drain lock prevents conflicting processes from trying to drain the same object at the same time.

Processes that drain only writers can run concurrently, but a process that drains all claim classes cannot drain an object concurrently with any other process. To drain an object, a

drainer first acquires one or more drain locks on the object, one for each claim class that it needs to drain.(一个claim分配一个drain lock)

When the locks are in place, the drainer can begin after all processes with claims on the object have released their claims. A drain lock also prevents new claimers from accessing an object while a drainer has control of it.

Three types of drain locks on an object correspond to the three claim classes:

In general, after an initial claim has been made on an object by a user, no other user in the system needs a drain lock. When the drain lock is granted, no drains on the object are in

process for the claim class needed, and the claimer can proceed.

The claimer of an object requests a drain lock in the following exceptional case: The claim is the first claim on an object before its data set has been physically opened. Here, acquiring the

drain lock ensures that no exception states prohibit allocating the data set.

For data sharing, when the claimer gets the drain lock, it makes its claim and releases the lock before beginning its processing.