Save-while-active function

 

The save-while-active function is an option on several i5/OS^® save commands. It allows you to save parts of your system without putting your system in a restricted state.

You can use the save-while-active function to reduce your save outage or to eliminate your save outage.

• Checkpoint processing with save-while-active
  Checkpoint processing occurs after the system determines exactly which objects it will save for a particular library. If the save-while-active request is for multiple libraries, then the system performs checkpoint processing for all libraries in the save request.

• Timestamp processing with save-while-active
  The save-active-time for an object can be useful when you determine which recovery procedures to use after you restore objects from the media.

• Commitment control with save-while-active
  This information applies if you are using commitment control and save-while-active to eliminate your save-outage time. This information applies only if you are not specifiying *NOCMTBDY for handling pending record changes on the SAVACTWAIT parameter.

 

Parent topic:

Save-while-active and your backup and recovery strategy

Related concepts
Save-while-active restrictions

 

How it works

i5/OS objects consist of units of storage, which are called pages. When you use the save-while-active function to save an object, the system creates two images of the pages of the object:

• The first image contains the updates to the object with which normal system activity works.

• The second image is an image of the object at a single point in time. The save-while-active job uses this image to save the object to the media.

In other words, when an application makes changes to an object during a save-while-active job, the system uses one image of the object's pages to make the changes. At the same time, the system uses the other image to save the object to the media. The image that the system saves does not have the changes you made during the save-while-active job. The image on the media is as it existed when the system reached a checkpoint.

 

Checkpoints

The checkpoint for an object is the instant in time that the system creates an image of that object. The image that the system creates at that instant in time is the checkpoint image of the object.

Creating a checkpoint image is similar to taking a photograph of a moving automobile. The point in time that you took the photograph would equate to the checkpoint. The photograph of the moving automobile would equate to the checkpoint image. When the system has finished making the checkpoint image of the object, the object has reached a checkpoint.

Despite the name save-while-active, you cannot change objects while the system obtains their checkpoint images. The system allocates (or locks) objects as it obtains checkpoint images. After the system obtains the checkpoint images, you can change the objects.

 

Synchronization

When you save more than one object, choose when the objects will reach a checkpoint in relationship to each other. This is synchronization. There are three kinds of synchronization:

• Full synchronizationWith full synchronization, the checkpoints for all of the objects occur at the same time. The checkpoints occur during a time period in which no changes can occur to the objects. IBM^® strongly recommends that you use full synchronization, even when you are saving objects in only one library.

• Library synchronizationWith library synchronization, the checkpoints for all of the objects in a library occur at the same time.

• System-defined synchronizationWith system-defined synchronization, the system decides when the checkpoints for the objects occur. The checkpoints for the objects may occur at different times resulting in complex restore procedures.

 

Save-outage time

The amount of time during the backup process that you cannot use the system is the save-outage time. You can use the save-while-active function to reduce or eliminate your save outage.

The easiest and recommended way to use the save-while-active function is to reduce your save-outage time. You can reduce your save-outage time by ending your applications that change objects. You can restart the applications after the system has reached a checkpoint for those objects. You can choose to have the save-while-active function send a notification when it completes the checkpoint processing. After the save-while-active function completes checkpoint processing, it is safe to start your applications again. When you use the save-while-active function in this way, the save-outage time can be much less than with normal save operations.

You can also use the save-while-active function to eliminate your save-outage time. When you use the save-while-active function to eliminate your save-outage time, you do not end the applications that make changes to the objects you save. However, the save operation affects the performance and response time of your applications. You should also use journaling or commitment control for all of the objects you are saving when using save-while-active in this way. Using the save-while-active function to eliminate your save-outage time may also greatly increase the complexity of your recovery procedures.

 

Save-while-active commands

The save-while-active function is an option on the i5/OS save commands listed below:

Command	Function
SAVLIB	Save Library
SAVOBJ	Save Object
SAVCHGOBJ	Save Changed Objects
SAVDLO	Save Document Library Objects
SAV	Save
SAVRSTLIB	Save/Restore Library
SAVRSTOBJ	Save/Restore Object
SAVRSTCHG	Save/Restore Changed Objects
SAVRSTDLO	Save/Restore Document Library Objects
SAVRST	Save/Restore