Document revision date: 19 July 1999

Page, swap, and dump files are created by default. However, you should understand these files. In addition, you might want to change them to meet the needs of your site.

Information Provided in This Chapter

This chapter describes the following tasks:

Task	Section
Displaying information about page and swap files	Section 15.3
Calculating appropriate sizes for files	Section 15.4
Minimizing dump file size when disk space is insufficient	Section 15.5
Writing the dump file to a device other than the system disk	Section 15.6
Using SDA to analyze the contents of a crash dump	Section 15.7
++Using SDA CLUE commands to obtain and analyze summary crash dump information	Section 15.8
+Using CLUE to obtain historical information about crash dumps	Section 15.9
Saving the contents of the system dump file after a system failure	Section 15.10
Copying dump files to tape or disk	Section 15.11
Freeing dump information from the page file	Section 15.12
Installing page and swap files	Section 15.13
Removing page, swap, and dump files	Section 15.14
Creating and modifying page, swap, and dump files	Section 15.15

This chapter explains the following concepts:

Concept	Section
Understanding dump files	Section 15.1
Understanding page and swap files	Section 15.2
Understanding the order of information in a selective system dump	Section 15.5.1
++Understanding SDA CLUE	Section 15.8.1
+Understanding CLUE	Section 15.9.1

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.

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 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 Compaq 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 furnished as an empty file in the operating system distribution kit. (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 Section 15.5 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 Section 15.15.1. You can place the system dump file on a disk other than the system disk. For more information, see Section 15.6.

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 and Alpha systems have the following differences:

• On Alpha 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 last 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):
  • With DOSD, error logs are written to a stub error log dump file, SYSDUMP.DMP, on the system disk. Also, both the error logs and system memory are written to the SYSDUMP.DMP file on the DOSD disk.
  • Without DOSD, both the error log and system memory are written to the SYSDUMP.DMP file on the system disk.
  
  On VAX systems with or without DOSD, the last system crash dump is always overwritten when an operator initiates a shutdown.

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

For detailed instructions, see Section 15.12.

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.

The two types of system dumps are physical and selective. Table 15-1 defines physical and selective system dumps. Table 15-3 compares the information available in physical and selective system dump files.

Table 15-1 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 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 Section 15.5 and the 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 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 Section 15.10 and Section 15.12, be sure to protect the copy from world read access. For more information about file protection, refer to the OpenVMS Guide to System Security.

15.2 Understanding Page and Swap Files

As part of memory management, the operating system makes efficient use of physical memory by moving information between physical memory and files stored on disk. The system does this in two ways: paging and swapping. Table 15-2 defines these and related terms.

Table 15-2 Paging and Swapping Terminology

Term	Definition
Paging	A memory management operation that provides the efficient use of physical memory allotted to a process. Paging moves infrequently used portions of a process workspace out of physical memory to a file. For more information about paging, refer to the OpenVMS Performance Management.
Page file	The file to which the system writes paged portions of memory. Your distribution kit includes a page file named SYS$SYSTEM:PAGEFILE.SYS. If necessary, you can use SYS$SYSTEM:PAGEFILE.SYS in place of the system crash dump file. For more information, see Section 15.1.1.
Swapping	A memory management operation that provides the efficient use of physical memory available for the entire system. Swapping moves the entire workspace of a less active process out of physical memory to a file. For more information about swapping, refer to the OpenVMS Performance Management.
Swap file	The file to which the system writes swapped portions of memory. Your distribution kit includes a swap file named SYS$SYSTEM:SWAPFILE.SYS.
Primary page and swap files	The default page and swap files provided with your distribution kit. These files are named SYS$SYSTEM:PAGEFILE.SYS and SYS$SYSTEM:SWAPFILE.SYS.
Secondary page and swap files	Additional page and swap files that you might create for performance or disk space reasons. If you kept the primary page and swap file on the system disk, the system uses the space in the secondary files for paging and swapping in addition to the space in the primary page and swap files. For information about creating secondary page and swap files, see Section 15.15.

Installing Files

Page and swap files must be installed before the system can use them. The system automatically installs the latest versions of SYS$SYSTEM:PAGEFILE.SYS and SWAPFILE.SYS during startup. If you create secondary page and swap files, you must make sure the system installs them during startup. For more information about installing page and swap files, see Section 15.13.

File Sizes and Locations

AUTOGEN automatically determines appropriate sizes for the files for your hardware configuration and system parameters. For special configurations or varying work loads, you might want to change the size of the page or swap file. For information, see Section 15.15.1.

If your system does not require the page file for storing system crash dumps, you can move it off the system disk. However, you should keep one page file on the system disk, if possible, so that you can boot the system if another disk holding the page files becomes unavailable. The swap file can also be moved off the system disk.

15.3 Displaying Information About Page and Swap Files

The DCL command SHOW MEMORY/FILES displays information about the page and swap files existing on your system, including file names, sizes, and the amount of space used. For example:

$ SHOW MEMORY/FILES System Memory Resources on 12-MAY-1998 11:54:20.06 Paging File Usage (pages): Free Reservable Total DISK$PAGE:[SYSEXE]SWAPFILE_IPL31.SYS;2 79992 79992 79992 DISK$PAGE:[SYSEXE]PAGEFILE_IPL31.SYS;1 23263 -370027 249992

Note that the number displayed in the column labeled "Reservable" can be a negative number. Processes can reserve more space than is available because it is unlikely that all the reserved space will be used for paging at one time.

15.4 Manually Calculating Appropriate Sizes for Dump, Page, and Swap Files

When you install or upgrade the operating system, AUTOGEN automatically calculates appropriate sizes for your system; these sizes are based on your hardware configuration and your system parameters. However, you can manually calculate the sizes for these files. The following sections describe how to determine appropriate sizes for the system page, swap, and dump files.

15.4.1 Calculating System Dump File Size

Sufficient space in the system dump file is critical to saving a complete crash dump. The AUTOGEN command procedure calculates an appropriate size for your system dump file. However, if you want to manually calculate the system dump file size, use the following formula, which calculates the file size required to hold a physical dump.

For SYSDUMP.DMP

On VAX systems, use the following formula:

size-in-blocks(SYS$SYSTEM:SYSDUMP.DMP) = size-in-pages(physical-memory) + number-of-error-log-buffers * blocks-per-buffer + 1

On Alpha systems, use the following formula:

size-in-blocks(SYS$SYSTEM:SYSDUMP.DMP) = size-in-pages(physical-memory) * blocks-per-page + number-of-error-log-buffers * blocks-per-buffer + size-in-pages (physical memory)/512 + 2

where:

size-in-pages	Is the size of physical memory, in pages. Use the DCL command SHOW MEMORY to determine the total size of physical memory on your system.
blocks-per-page	Is the number of blocks per page of memory. On Alpha systems, calculate the number of blocks per page of memory by dividing the system's page size by 512 (the size of a block). Use the following commands: $ PAGESIZE==F$GETSYI ("PAGE_SIZE") $ BLOCKSPERPAGE=PAGESIZE/512 $ SHOW SYMBOL BLOCKSPERPAGE
number-of-error-log-buffers	Is the value of the system parameter ERRORLOGBUFFERS. This parameter sets the number of error log buffers to permanently allocate in memory.
blocks-per-buffer	Is the value of the system parameter ERLBUFFERPAGES. This parameter sets the number of pages of memory in each buffer.

A large memory system or a system with small disk capacity might not be able to supply enough disk space for a full memory dump. Under these circumstances, you should set the system parameter DUMPSTYLE to the appropriate value to indicate that the system is to dump only selective information. For more information, see Section 15.5.

For PAGEFILE.SYS

If SYS$SYSTEM:SYSDUMP.DMP does not exist, the system writes crash dumps to the primary page file SYS$SYSTEM:PAGEFILE.SYS. The AUTOGEN command procedure calculates an appropriate size for your page file. However, to manually calculate the minimum page file size required to hold crash dumps, use the following formula:

On VAX systems:

size-in-blocks(SYS$SYSTEM:PAGEFILE.SYS) = size-in-pages(physical-memory) + number-of-error-log-buffers * blocks-per-buffer + 1 + 1000

On Alpha systems:

size-in-blocks(SYS$SYSTEM:PAGEFILE.SYS) = size-in-pages(physical-memory) * blocks-per-page + number-of-error-log-buffers * blocks-per-buffer + size-in-pages (physical memory)/512 + 2 + value of the system parameter RSRVPAGCNT

where:

size-in-pages	Is the size of physical memory, in pages. Use the DCL command SHOW MEMORY to determine the total size of physical memory on your system.
blocks-per-page	Is the number of blocks per page of memory. On Alpha systems, calculate the number of blocks per page of memory by dividing the system's page size by 512 (the size of a block). Use the following commands: $ PAGESIZE==F$GETSYI ("PAGE_SIZE") $ BLOCKSPERPAGE=PAGESIZE/512 $ SHOW SYMBOL BLOCKSPERPAGE
number-of-error-log-buffers	Is the value of the system parameter ERRORLOGBUFFERS. This parameter sets the number of error log buffers to permanently allocate in memory.
blocks-per-buffer	Is the value of the system parameter ERLBUFFERPAGES. This parameter sets the number of pages of memory in each buffer.
RSRVPAGCNT	Is the value of the RSRVPAGCNT special system parameter.

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