Understanding Dump Files

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HP OpenVMS System Manager's Manual, Volume 2:...

Managing Page, Swap, and Dump Files

Understanding Page and Swap Files

Understanding Dump Files

When the operating system detects an unrecoverable error or an inconsistency within itself that causes the system to fail, it writes the contents of the error log buffers, processor registers, and memory into the system dump file, overwriting its previous contents.

The contents of error log buffers are also written to the error log dump file. The error log dump file is provided so that the system can be updated on reboot to include error log entries that were created but not written at the time of a system crash.

System Dump File

When writing the system dump file, the system displays console messages and information about the error or inconsistency. The last message tells you that the dump file was successfully written.

Be sure to wait until you see the termination message before using the console terminal to halt the system. If you do not wait, your system might not save a complete system dump file.

Console messages and the system dump file are important sources of information in determining the cause of a system failure. Use the contents in the following ways:

Use the System Dump Analyzer utility (SDA) to analyze the contents of the dump and determine the cause of a failure.

On Alpha and I64 systems, use SDA CLUE commands to obtain and analyze summary dump file information.

On VAX systems, use CLUE to obtain historical information from system dump files.

Contact your HP support representative, indicating that you have made a copy of the contents of the system dump.

The default system dump file, SYS$SPECIFIC:[SYSEXE]SYSDUMP.DMP, is created during installation. (You do not need a system dump file to run the operating system. However, you must have a system dump file to diagnose system crashes.) AUTOGEN automatically determines an appropriate size for the system dump file for your hardware configuration and system parameters. Refer to Minimizing System Dump File Size When Disk Space Is Insufficient for information about minimizing system dump file size if disk space is insufficient.

For special configurations or varying work loads, you can change the size of the system dump file. For information, see Using AUTOGEN (Recommended Method). You can write the system dump file on a disk other than the system disk. This is referred to as dump off system disk (DOSD). For more information, see Writing the System Dump File to an Alternate Disk.

Error Log Dump File

AUTOGEN creates the error log dump file during installation; its size depends on your configuration and system parameters. Error log dump files on VAX, Alpha, and I64 systems have the following differences:

On Alpha and I64 systems, the error log dump file is called SYS$ERRLOG.DMP; it is located on the system disk. When an operator initiates a shutdown, the system writes the contents of error log buffers to the error log dump file and not to the system dump file. Thus, the previous system crash dump is not overwritten.

On VAX systems, the error log dump file is called SYSDUMP.DMP. How the system handles this file depends on whether you are using dump off system disk (DOSD):
On VAX systems with or without DOSD, the previous system crash dump is always overwritten when an operator initiates a shutdown.

Using the Page File to Store System Crash Dumps

The operating system uses the latest version of SYS$SYSTEM:SYSDUMP.DMP to store system crash dumps. If SYSDUMP.DMP does not exist in SYS$SYSTEM, the operating system uses the system page file, SYS$SYSTEM:PAGEFILE.SYS, overwriting the contents of that file.

If the SAVEDUMP system parameter is set, the crash dump is retained in PAGEFILE.SYS when the system is booted. If SAVEDUMP is clear, the system uses the page file for paging; any dump written to the page file is lost.

If you use SYS$SYSTEM:PAGEFILE.SYS to capture system crash dumps, you should later free the space occupied by the dump for use in system paging, with either of the following methods:

Use the SDA COPY command to copy the dump from the page file to a different file.

Use the SDA RELEASE command to delete the information from the page file.

For detailed instructions, see Freeing Dump Information from the Page File.

Include the appropriate commands in the SYSTARTUP_VMS.COM startup command procedure to free dump information from the page file each time the system reboots.

Be careful when using the page file for selective dumps. Selective dumps use up all available space. If your page file is small, selective dump information might fill the entire page file, leaving no space for paging during system boot. This can cause the system to hang during reboot.

Understanding Types of System Dumps

The two types of system dumps are physical and selective. Definitions of Physical and Selective System Dumps defines physical and selective system dumps. Comparison of Physical and Selective System Dump Files compares the information available in physical and selective system dump files.

Table 1 Definitions of Physical and Selective System Dumps
Type Description

Physical dump
Writes the entire contents of physical memory to the system dump file. To ensure a useful physical dump, the system dump file must be large enough to contain all of physical memory.

Selective dump
Stores those portions of memory most likely to be useful in crash dump analysis. A selective system dump is useful when disk space is not available to hold all of physical memory.

**Table 1** **Definitions of Physical and Selective System Dumps**
Type	Description
Physical dump	Writes the entire contents of physical memory to the system dump file. To ensure a useful physical dump, the system dump file must be large enough to contain all of physical memory.
Selective dump	Stores those portions of memory most likely to be useful in crash dump analysis. A selective system dump is useful when disk space is not available to hold all of physical memory.

Requirements for Creating a Useful System Dump

The following requirements must be met for the operating system to write a useful system dump file:

The system parameter DUMPBUG must be set to 1 (the default value).

If the system parameter SAVEDUMP is set to 0 (the default) the file SYS$SPECIFIC:[SYSEXE]SYSDUMP.DMP must exist on the system disk.

If the file SYS$SPECIFIC:[SYSEXE]SYSDUMP.DMP does not exist on the system disk, the page file must be used to store the dump. The system parameter SAVEDUMP must be set to 1 and the file SYS$SPECIFIC:[SYSEXE]PAGEFILE.SYS must exist on the system disk.

If the file SYS$SPECIFIC:[SYSEXE]SYSDUMP.DMP does not exist and SYS$SPECIFIC:[SYSEXE]PAGEFILE.SYS is not available for system dumps, you must create the dump file on an alternate disk (see Writing the System Dump File to an Alternate Disk).

If sufficient disk space is not available to allow a system dump file that can hold all of memory, the system parameter DUMPSTYLE must be set to the appropriate value to store a selective system dump. For more information, refer to Minimizing System Dump File Size When Disk Space Is Insufficient and the HP OpenVMS System Management Utilities Reference Manual.

The system dump file (or page file if the SAVEDUMP system parameter is set) must be large enough to hold all information that is to be written if the system fails.

If the system parameter DUMPBUG is set, AUTOGEN automatically sizes SYSDUMP.DMP if enough disk space is available.

If the system parameter SAVEDUMP is set, AUTOGEN performs no operations on the system dump file.

AUTOGEN sizes the page file only for paging use, regardless of whether the SAVEDUMP system parameter is set.

BACKUP Considerations

A system dump file has the NOBACKUP attribute; therefore, the Backup utility (BACKUP) does not copy the file unless you use the qualifier /IGNORE=NOBACKUP when invoking BACKUP. When you use the SDA COPY command to copy the system dump file to another file, the operating system does not automatically set the new file to NOBACKUP. If you want to set the NOBACKUP attribute on the copy, use the SET FILE command with the /NOBACKUP qualifier as described in the HP OpenVMS DCL Dictionary.

Security Considerations

By default, SYS$SYSTEM:SYSDUMP.DMP is protected against world access. Because a system dump file can contain privileged information, you should keep this level of protection on system dump files. Similarly, when you copy system dump files using the System Dump Analyzer utility (SDA) as explained in Saving the Contents of the System Dump File After a System Failure and Freeing Dump Information from the Page File, be sure to protect the copy from world read access. For more information about file protection, refer to the HP OpenVMS Guide to System Security.

Understanding Page and Swap Files