DB2 claim與drain

DB2使用claim & drain來控制SQL與utility對數據庫對象的訪問。

claim說明有對象正在訪問或是將要訪問該對象，而drain則是在對象上面加一個drain lock，等全部的claimers執行完成之後，對該對象進行特定的處理操做。drain lock的目的是防止多個process同時drain一個對象，從而出現drain deadlock。

兩者都是對象級別的，它獨立於事務鎖而存在。

須要注意的drain是action，即它也是一個claim，它發出drain的指令，拿到drain lock，而後就等待其餘的claimers執行完成，而後執行本身的drained claim。？

 

Claims and drains for concurrency control

Claims and drains are taken at the object level only and not at the page or row level. DB2 utilities, commands, and some ALTER, CREATE, and DROP statements can take over

access to some DB2 objects independent of any transaction locks that are held on the object.

Claims and drains on an index unlike transaction locks, the claims and drains can be taken on indexes also. DB2 utilities, commands, and some ALTER, CREATE, and DROP statements can take over access to certain objects.

index上容許有drain和claim。

The following objects are subject to takeover by those operations:

• Simple and segmented table spaces

• Partitions of table spaces

• LOB table spaces

• XML table spaces

• Non-partitioned index spaces

• Partitions of index spaces

• Logical partitions of non-partitioned indexes

 

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的生命週期在一個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.

Three classes of claims

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容許原有claim一直活到commit point，但不容許新的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.

Drained claim classes

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).

How DB2 uses drain locks

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.(一個claim分配一個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.

Types of drain locks

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

•  Write

•  Repeatable read

•  Cursor stability read

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.