Warm Starts, Force Starts, Cold Starts, and Clean Starts

z/VM® provides you with four basic types of starts: warm, force, cold, and clean. The difference among them is in how much of the system's environment CP restores, with a warm start restoring the most and a clean start the least. You save the system's environment by entering the SHUTDOWN command. You restore it by bringing up CP with a warm start. If a warm start fails, contact your system support personnel before you use a force or cold start.

In order to restore the system's environment, you must save it first. As mentioned, you do this by entering a SHUTDOWN command. When you enter this command, CP saves the following parts of the system's environment:
  • Any accounting, EREP, and symptom records in storage.

    Accounting, EREP, and symptom record recording service virtual machines normally transfer these records from storage to disk. If these virtual machines are not retrieving, the records remain in storage. The SHUTDOWN command will write any records that are still in storage into the checkpoint area on DASD so that during the next IPL the records can be returned to storage for retrieval.

  • System log message.

    The system log message communicates information to users when they log on or reconnect.

  • Spool file queues.

    Spool files are collections of data on disk waiting to be processed by real or virtual readers, printers, or punches. The SHUTDOWN command saves information about the queues that locate these spool files.

    Note: Spool files may not be restored in the same order following a warm or force start.
  • The system data file queues.

    The system data file queues hold the following collections of data, called system data files: named saved systems (NSSs), discontiguous saved segments (DCSSs), image libraries, user class restructure (UCR) files, message repository files, and system trace files.

    Note: UCR files might still exist but are no longer supported and will not be processed by CP.
    • A named saved system (NSS) is a copy of an operating system that a user has named and retained in a system data file. The user can load the operating system by its name, which is more efficient than loading it by device number.
    • A discontiguous saved segment (DCSS) refers to one or more pages of storage that a user has named and retained in a system data file. When a discontiguous saved segment, defined as shareable, is loaded, more than one user can access it.
    • An image library is a set of modules, contained in a system data file, that define the spacing, characters, and copy-modification data that a printer uses to print a spool file.
    • Message repository files contain z/VM messages and responses translated into a national language.
    • System trace files contain records of events that occur within the system. Use these files to determine the source of problems in the system.

    The SHUTDOWN command saves information about the queues that locate these system data files.

  • The status of unit record devices, displays, and 3270 printers.

When you enter the SHUTDOWN command, you save all these parts of the system's environment. Then, when you bring the system back up with a warm start, CP restores them.

If you enter SHUTDOWN REIPL, CP attempts to do an automatic start of the specified module.

See the z/VM: CP Commands and Utilities Reference for other restrictions when you re-IPL another module.

Unless you have a reason to do otherwise, specify a warm start. Keep in mind that if you do not shut down the z/VM system with the SHUTDOWN command, you may not be able to bring it back up with a warm start. The information that the SHUTDOWN command saves is what allows a warm start to work.

During a force start, CP tries to restore most of what it restores during a warm start, but it may not be able to do so completely. Just as in a warm start, CP tries to restore, in this order:

  1. Accounting, EREP, and symptom records in storage
  2. System log message
  3. Spool file and system data file queues.

However, if CP encounters an error during the first step, it immediately goes on to recover the spool file and system data file queues. Unless you specified FEATURES ENABLE LOGMSG_FROM_FILE in the system configuration file, the system log message is lost. Any remaining accounting, EREP, and symptom records are lost. Further, if CP encounters an error while recovering a spool file, it immediately goes on to the next file in the queue, and the file on which the error occurred is scheduled to be deleted. When the preliminary phase of spooling initialization is complete, a message providing spool file summary status is displayed, and the operator can then stop system initialization without the loss of any spool files.

Also, a force start does not restore any of the unit record device characteristics, such as the class to be processed or the image library. Instead, a force start tries to start the unit record devices with the default characteristics specified on the RDEVICE statement in the system configuration file. A force start does try to restore the status of all displays and 3270 printers.

Therefore, after a force start, CP may not have completely recovered all of the system's environment. Use a force start only when authorized by the system support personnel after a warm start fails.

If a force start fails, you must perform a cold start. During a cold start, CP tries to recover only the system data files. This means that when you perform a cold start, you lose all of your spool files, all accounting, EREP, and symptom records in storage. The system log message is lost if you did not specify the FEATURES ENABLE LOGMSG_FROM_FILE statement in the system configuration file. The status of your unit record devices, displays, and 3270 printers may also differ from their status before the restart. Use a cold start only when both a warm start and a force start fail.

The essential points to grasp about warm starts, force starts, cold starts, and clean starts are:
  • To save the system's environment before you restart it, use the SHUTDOWN command.
  • To restore the system's environment, bring the system back up with a warm start.
  • If a warm start fails,
    • Check the DASD volumes.
    • Contact system support personnel.
    • When repeated attempts to perform a warm start fail, try a force start to restore as much of the system's environment as possible.
  • If you decide to continue with system initialization even though there are files scheduled to be deleted, remember that those files cannot be recovered.
  • If a force start fails, try a cold start.
  • A clean start IPLs the system without attempting to recover spool files and system data files that existed prior to system shutdown.

The next section explains how to specify each of the basic types of starts.