OpenVMS System Manager's Manual

Document revision date: 15 July 2002

OpenVMS System Manager's Manual

Contents

Index

16.1.1 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 Section 16.13.

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.

Caution

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.

16.1.2 Understanding Types of System Dumps

The two types of system dumps are physical and selective. Table 16-1 defines physical and selective system dumps. Table 16-3 compares the information available in physical and selective system dump files.

Table 16-1 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 16-1 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 Section 16.7).
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 16.5 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 16.11 and Section 16.13, be sure to protect the copy from world read access. For more information about file protection, refer to the OpenVMS Guide to System Security.

16.2 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 16-2 defines these and related terms.

Table 16-2 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 manual.

Page file The file to which the system writes paged portions of memory. The OpenVMS installation process creates 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 16.1.1.

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 manual.

Swap file The file to which the system writes swapped portions of memory. The OpenVMS installation procedure creates a swap file named SYS$SYSTEM:SWAPFILE.SYS.

Primary page and swap files The default page and swap files created during OpenVMS installation. 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 16.16.

**Table 16-2 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 manual.
Page file	The file to which the system writes paged portions of memory. The OpenVMS installation process creates 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 16.1.1.
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 manual.
Swap file	The file to which the system writes swapped portions of memory. The OpenVMS installation procedure creates a swap file named SYS$SYSTEM:SWAPFILE.SYS.
Primary page and swap files	The default page and swap files created during OpenVMS installation. These files are named SYS$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 16.16.

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 16.14.

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 16.16.1.

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.

16.3 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 19-JAN-2001 13:35:26.58 Swap File Usage (8KB pages): Index Free Size DISK$PAGE_DUMPS:[SYS0.SYSEXE]SWAPFILE.SYS;2 1 7992 8248 Paging File Usage (8KB pages): Index Free Size DISK$PAGE_DUMPS:[SYS0.SYSEXE]PAGEFILE.SYS;1 254 13722 16496 Total committed paging file usage: 4870

The total committed paging file usage is the number of pages in the system that require pagefile space for paging. It can be bigger than the total number of pagefile pages that are available because it is unlikely that all the required space will be used for paging at one time.

16.4 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.

16.4.1 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 + 10

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 pagelets (blocks) 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 16.5.

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 +10 + value of the system parameter RSRVPAGCNT

Caution

This formula calculates only the minimum size requirement for saving a dump in the system's primary page file. For most systems, the page file must be larger than this to avoid hanging the system. For more information about calculating the page file size, see Section 16.4.3.

16.4.2 Calculating Error Log Dump File Size

These calculations differ on OpenVMS VAX and Alpha systems:

On Alpha Systems

On Alpha systems, the AUTOGEN command procedure calculates the appropriate size of your error log dump file. However, to calculate the size of the file manually, use the following formula, which calculates the file size required to hold all the error log buffers:

size-in-blocks(SYS$SYSTEM:SYS$ERRLOG.DMP) = number-of-error-log-buffers * blocks-per-buffer + 2

where:

number-of-error-log-buffers Is the value of the system parameter ERRORLOGBUFFERS. This parameter sets the number of error log buffers that are permanently allocated in memory.

blocks-per-buffer Is the value of the system parameter ERLBUFFERPAGES. This parameter sets the number of pagelets (blocks) of memory in each buffer.

On VAX Systems

On VAX systems, the size of the error log dump file depends on your DOSD:

Without DOSD, error logs are written to the dump file SYSDUMP.DMP on the system disk. See Section 16.4.1 for sizing information.
With DOSD:
- Both error logs and system memory are written to the SYSDUMP.DMP file on the DOSD disk. See Section 16.4.1 for sizing information.
- In addition, error logs are written to the stub system dump file SYSDUMP.DMP on the system disk. Because this is a fixed-length file of 2048 blocks, no sizing calculations are required.

16.4.3 Calculating Page File Size

Sufficient page file space is critical to system performance. The AUTOGEN command procedure calculates an appropriate size for your page file space. The size calculated by AUTOGEN should be sufficient. However, to manually calculate the size for page file space, use one of the following formulas.

On VAX Systems

On VAX systems, use the following formula:

size-in-blocks (total for all page files on the system) = size-of-average-process (in pages) * maximum-number-of-processes

The size-of-average-process is the value of the average virtual size of the process. Use the following command to find it:
$ SHOW PROCESS/CONTINUOUS/ID=pid
Specify this value in pages.
The maximum-number-of-processes is the value of the MAXPROCESSCNT system parameter.

If the result of the formula is less than VIRTUALPAGECNT, use the value of VIRTUALPAGECNT instead.

To determine a system's virtual page count, enter the following command:

$ WRITE SYS$OUTPUT F$GETSYI ("VIRTUALPAGECNT")

On Alpha Systems

On Alpha systems, no simple formula can be given because of the wide variation possible in memory size and usage. You can use the following formula for systems up to 512MB:

size-in-blocks (total for all page files on the system) = size-of-average-process (in pages) * blocks-per-page * maximum-number-of-processes

The size-of-average-process is the value of the average virtual size of the process. Use the following command to find it:
$ SHOW PROCESS/CONTINUOUS/ID=pid
Specify this value in pages.
Calculate blocks-per-page by dividing the system page size by 512 (pagelet size). For example, a system with a page size of 8192 has 16 pagelets per page.
To determine a system's page size, enter the following command:
$ WRITE SYS$OUTPUT F$GETSYI ("PAGE_SIZE")
The maximum-number-of-processes is the value of the MAXPROCESSCNT system parameter.

For systems over 512MB, follow the steps described below for monitoring page file usage ( Section 16.4.3.2), and make adjustments as necessary.

16.4.3.1 Representing Page File Size

The page file size you calculate can be represented in one of the following ways:

In the primary page file only
Distributed across primary and secondary page files
If you have removed the primary page file in SYS$SYSTEM, distributed across a number of secondary page files

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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 pagelets (blocks) of memory in each buffer.