HP OpenVMS Systems Documentation

There are two types of dump files---a full memory dump (also known as a physical dump), and a dump of selected virtual addresses (also known as a selective dump). Both full and selective dumps may be produced in either compressed or uncompressed form. Compressed dumps save disk space and time taken to write the dump at the expense of a slight increase in time to access the dump with SDA. The SDA commands COPY/COMPRESS and COPY/DECOMPRESS can be used to convert an existing dump.

A dump can be written to the system disk, or to another disk set aside for dumps. When using a disk other than a system disk, the disk name is set in the console environment variable DUMP_DEV. This disk is also known as the "dump off system disk" (DOSD) disk.

When writing a system dump, information about the crash is displayed at the system console. This can be either minimal output (for example, bug check code, process name, and image name), or verbose output (for example, executive layout, stack and register contents).

In an OpenVMS Alpha Galaxy system, shared memory is dumped by default. It is sometimes necessary to disable the dumping of shared memory. For more information about shared memory, see OpenVMS Alpha Galaxy Guide.

DUMPSTYLE, which specifies the method of writing system dumps, is a 32-bit mask. Table 2-1 shows how the bits are defined. Each bit can be set independently. The value of the SYSGEN parameter is the sum of the values of the bits that have been set. Remaining or undefined values are reserved to Compaq.

The default setting for DUMPSTYLE is 0 (an uncompressed full dump, including shared memory, written to the system disk). Unless a value for DUMPSTYLE is specified in MODPARAMS.DAT, AUTOGEN.COM will set DUMPSTYLE either to 1 (an uncompressed selective dump, including shared memory, written to the system disk) if there is less than 128 megabytes of memory on the system, or to 9 (a compressed selective dump, including shared memory, written to the system disk).

2.2.1.2 Comparison of Full and Selective Dumps

A full dump requires that all physical memory be written to the dump file. This ensures the presence of all the page table pages required for SDA to emulate translation of system virtual addresses. Any even-numbered value in the DUMPSTYLE system parameter generates a full dump.

In certain system configurations, it may be impossible to preserve the entire contents of memory in a disk file. For instance, a large memory system or a system with small disk capacity may not be able to supply enough disk space for a full memory dump. If the system dump file cannot accommodate all of memory, information essential to determining the cause of the system failure may be lost.

To preserve those portions of memory that contain information most useful in determining the causes of system failures, a system manager sets the value of the DUMPSTYLE system parameter to specify a dump of selected virtual address spaces. In a selective dump, related pages of virtual address space are written to the dump file as units called logical memory blocks (LMBs). For example, one LMB consists of the page tables for system space; another is the address space of a particular process. Those LMBs most likely to be useful in crash dump analysis are written first. Any odd-numbered value in the DUMPSTYLE system parameter generates a selective dump.

Table 2-2 compares full and selective style dumps.

You can adjust the size of the system page file and dump file using AUTOGEN (the recommended method) or by using SYSGEN.

AUTOGEN automatically calculates the appropriate sizes for page and dump files. AUTOGEN invokes the System Generation utility (SYSGEN) to create or change the files. However, you can control sizes calculated by AUTOGEN by defining symbols in the MODPARAMS.DAT file. The file sizes specified in MODPARAMS.DAT are copied into the PARAMS.DAT file during AUTOGEN's GETDATA phase. AUTOGEN then makes appropriate adjustments in its calculations.

Although Compaq recommends using AUTOGEN to create and modify page and dump file sizes, you can use SYSGEN to directly create and change the sizes of those files.

The sections that follow discuss how you can calculate the size of a dump file.

See the OpenVMS System Manager's Manual for detailed information about using AUTOGEN and SYSGEN to create and modify page and dump file sizes.

2.2.1.4 Writing to the System Dump File

OpenVMS Alpha writes the contents of the error-log buffers, processor registers, and memory into the system dump file, overwriting its previous contents. If the system dump file is too small, OpenVMS Alpha cannot copy all memory to the file when a system failure occurs.

SYS$SYSTEM:SYSDUMP.DMP (SYS$SPECIFIC:[SYSEXE]SYSDUMP.DMP) is created during installation. To successfully store a crash dump, SYS$SYSTEM:SYSDUMP.DMP must be enlarged to hold all of memory (full dump) or all of system space and the key processes (selective dump).

To calculate the correct size for an uncompressed full dump to SYS$SYSTEM:SYSDUMP.DMP, 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

Use the DCL command SHOW MEMORY to determine the total size of physical memory on your system. There is a variable number of error log buffers in any given system, depending on the setting of the ERRORLOGBUFFERS system parameter. The size of each buffer depends on the setting of the ERLBUFFERPAGES parameter. (See the OpenVMS System Manager's Manual for additional information about these parameters.)

2.2.1.5 Writing to the Dump File off the System Disk

OpenVMS Alpha allows you to write the system dump file to a device other than the system disk. This is useful in large memory systems and in clusters with common system disks where sufficient disk space, on one disk, is not always available to support customer dump file requirements. To perform this activity, the DUMPSTYLE system parameter must be correctly enabled to allow the bugcheck code to write the system dump file to an alternative device.

The requirements for writing the system dump file off the system disk are the following:

• The dump device directory structure must resemble the current system disk structure. The [SYSn.SYSEXE]SYSDUMP.DMP file will reside there, with the same boot time system root.
  You can use AUTOGEN to create this file. In the MODPARAMS.DAT file, the following symbol prompts AUTOGEN to create the file:

  DUMPFILE_DEVICE = $nnn$ddcuuuu

• The dump device cannot be part of a volume set or a member of a shadow set.
• You must set DOSD for SDA CLUE as described in Chapter 5.
• The DUMP_DEV environment variable must exist on your system. You specify the dump device at the console prompt, using the following format: >>>SET DUMP_DEV device-name[,...]
  On some CPU types, you can enter a list of devices. The list can include various alternate paths to the system disk and the dump disk.
  By specifying alternate paths in DUMP_DEV, a dump can still be written if the disk fails over to an alternate path while the system is running. When the system crashes, the bugcheck code can use the alternate path by referring to the contents of DUMP_DEV.
  When you enter a list of devices, however, the system disk must come last.

For information on how to write the system dump file to an alternative device to the system disk, see the OpenVMS System Manager's Manual, Volume 2: Tuning, Monitoring, and Complex Systems.

2.2.1.6 Writing to the System Page File

If SYS$SYSTEM:SYSDUMP.DMP does not exist, and there is no DOSD device or dump file, the operating system writes the dump of physical memory into SYS$SYSTEM:PAGEFILE.SYS, the primary system page file, overwriting the contents of that file.

If the SAVEDUMP system parameter is set, the dump file is retained in PAGEFILE.SYS when the system is booted after a system failure. If the SAVEDUMP parameter is not set, which is the default, OpenVMS Alpha uses the entire page file for paging and any dump written to the page file is lost. (To examine or change the value of the SAVEDUMP parameter, consult the OpenVMS System Manager's Manual, Volume 2: Tuning, Monitoring, and Complex Systems.)

To calculate the minimum size for a full memory dump to SYS$SYSTEM:PAGEFILE.SYS, use the following formula:

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 * blocks-per-page

Note that this formula calculates the minimum size requirement for saving a physical dump in the system's page file. Compaq recommends that the page file be a bit larger than this minimum to avoid hanging the system. Also note that you can only write the system dump into the primary page file (SYS$SYSTEM:PAGEFILE.SYS). Secondary page files cannot be used to save dump file information.

Note also that OpenVMS will not fill the page file completely when writing a system dump, since the system might hang when rebooting after a system crash. RSRVPAGCNT pages are kept unavailable for dumps. This applies to both full dumps and selective dumps.

Writing crash dumps to SYS$SYSTEM:PAGEFILE.SYS presumes that you will later free the space occupied by the dump for use by the pager. Otherwise, your system may hang during the startup procedure. To free this space, you can do one of the following:

• Include SDA commands that free dump space in the site-specific startup command procedure (described in Section 2.2.3).
• Use the SDA COPY command to copy the dump from SYS$SYSTEM:PAGEFILE.SYS to another file. Use the SDA COPY command instead of the DCL COPY command because the SDA COPY command only copies the blocks used by the dump and causes the pages occupied by the dump to be freed from the system's page file.
• If you do not need to copy the dump elsewhere, issue an ANALYZE/CRASH_DUMP/RELEASE command. When you issue this command, SDA immediately releases the pages to be used for system paging, effectively deleting the dump. Note that this command does not allow you to analyze the dump before deleting it.

Every time the operating system writes information to the system dump file, it writes over whatever was previously stored in the file. The system writes information to the dump file whenever the system fails. For this reason, the system manager must save the contents of the file after a system failure has occurred.

The system manager can use the SDA COPY command or the DCL COPY command. Either command can be used in a site-specific startup procedure, but the SDA COPY command is preferred because it marks the dump file as copied. As mentioned earlier, this is particularly important if the dump was written into the page file, SYS$SYSTEM:PAGEFILE.SYS, because it releases those pages occupied by the dump to the pager. Another advantage of using the SDA COPY command is that this command copies only the saved number of blocks and not necessarily the whole allotted dump file. For instance, if the size of the SYSDUMP.DMP file is 100,000 blocks and the bugcheck wrote only 60,000 blocks to the dump file, then DCL COPY would create a file of 100,000 blocks. However, SDA COPY would generate a file of only 60,000 blocks.

Because system dump files are set to NOBACKUP, the Backup utility (BACKUP) does not copy them to tape 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, OpenVMS Alpha does not set the new file to NOBACKUP.

As created during installation, the file SYS$SYSTEM:SYSDUMP.DMP is protected against world access. Because a dump file can contain privileged information, Compaq recommends that the system manager does not change this default protection.

2.2.3 Invoking SDA When Rebooting the System

When the system reboots after a system failure, SDA is automatically invoked by default. SDA archives information from the dump in a history file. In addition, a listing file with more detailed information about the system failure is created in the directory pointed to by the logical name CLUE$COLLECT. (Note that the default directory is SYS$ERRORLOG unless you redefine the logical name CLUE$COLLECT in the procedure SYS$MANAGER:SYLOGICALS.COM.) The file name is in the form CLUE$node_ddmmyy_hhmm.LIS where the timestamp (hhmm) corresponds to the system failure time and not the time when the file was created.

Directed by commands in a site-specific file, SDA can take additional steps to record information about the system failure. They include the following:

• Copying the contents of the dump file to another file. This information is otherwise lost at the next system failure when the system saves information only about that failure.
• Supplementing the contents of the list file containing the output of specific SDA commands.
  If the logical name CLUE$SITE_PROC points to a valid and existing command file, it will be executed as part of the CLUE HISTORY command when you reboot. If used, this file should contain only valid SDA commands.
  Generated by a set sequence of commands, the CLUE list file contains only an overview of the failure and is unlikely to provide enough information to determine the cause of the failure. Compaq, therefore, recommends that you always copy the dump file.

The following example shows SDA commands that can make up your site-specific command file to produce a more complete SDA listing after each system failure, and to save a copy of the dump file:

!
! SDA command file, to be executed as part of the system
! bootstrap from within CLUE. Commands in this file can
! be used to save the dump file after a system bugcheck, and
! to execute any additional SDA commands.
!

! Note that the logical name DMP$ must have been defined
! within SYS$MANAGER:SYLOGICALS.COM
!
READ/EXEC                   ! read in the executive images' symbol tables
SHOW STACK                  ! display the stack
COPY DMP$:SAVEDUMP.DMP      ! copy and save dump file
!

The CLUE HISTORY command is executed first, followed by the SDA commands in this site-specific command file. See the reference section on CLUE HISTORY for details on the summary information that is generated and stored in the CLUE list file by the CLUE HISTORY command. Note that the SDA COPY command is final command. If the dump has been written to PAGEFILE.SYS, then the space used by the dump will be automatically returned for use for paging as soon as the COPY is complete and no more analysis is possible.

To point to your site-specific file, add a line such as the following to the file SYS$MANAGER:SYLOGICALS.COM:

$ DEFINE/SYSTEM CLUE$SITE_PROC SYS$MANAGER:SAVEDUMP.COM

In this example, the site-specific file is named SAVEDUMP.COM.

The CLUE list file can be printed immediately or saved for later examination.

SDA is invoked and executes the specified commands only when the system boots for the first time after a system failure. If the system is booting for any other reason (such as a normal system shutdown and reboot), SDA exits.

If CLUE files occupy more space than the threshold allows (the default is 5000 blocks), the oldest files will be deleted until the threshold limit is reached. The threshold limit can be customized with the CLUE$MAX_BLOCK logical name.

To prevent the running of CLUE at system startup, define the logical CLUE$INHIBIT in the SYLOGICALS.COM file as TRUE in the system logical name table.

2.3 Analyzing a System Dump

SDA performs certain tasks before bringing a dump into memory, presenting its initial displays, and accepting command input. These tasks include the following:

• Verifying that the process invoking it is suitably privileged to read the dump file
• Using RMS to read in pages from the dump file
• Building the SDA symbol table from the files SDA$READ_DIR:SYS$BASE_IMAGE.EXE and SDA$READ_DIR:REQSYSDEF.STB
• Executing the commands in the SDA initialization file

For detailed information on investigating system failures, see Section 2.7.

2.3.1 Requirements

To analyze a dump file, your process must have read access both to the file that contains the dump and to copies of SDA$READ_DIR:SYS$BASE_IMAGE.EXE and SDA$READ_DIR:REQSYSDEF.STB (the required subset of the symbols in the file SYSDEF.STB). SDA reads these tables by default.

2.3.2 Invoking SDA

If your process can access the files listed in Section 2.3.1, you can issue the DCL command ANALYZE/CRASH_DUMP to invoke SDA. If you do not specify the name of a dump file in the command, SDA prompts you:

$ ANALYZE/CRASH_DUMP
_Dump File:

The default file specification is as follows:

SYS$DISK:[default-dir]SYSDUMP.DMP

SYS$DISK and [default-dir] represent the disk and directory specified in your last SET DEFAULT command.

If you are rebooting after a system failure, SDA is automatically invoked. See Section 2.2.3.

2.3.3 Mapping the Contents of the Dump File

SDA first attempts to map the contents of memory as stored in the specified dump file. To do this, it must first locate the page tables for system space among its contents. The system page tables contain one entry for each page of system virtual address space.

• If SDA cannot find the system page tables in the dump file, it displays the following message:

  %SDA-E-SPTNOTFND, system page table not found in dump file

  
  If that error message is displayed, you cannot analyze the crash dump, but must take steps to ensure that any subsequent dump can be analyzed. To do this, you must either adjust the DUMPSTYLE system parameter as discussed in Section 2.2.1.1 or increase the size of the dump file as indicated in Section 2.2.1.3.
• If SDA finds the system page tables in an incomplete dump, the following message is displayed:

  %SDA-W-SHORTDUMP, dump file was n blocks too small when dump written; analysis may not be possible

Under certain conditions, some memory locations might not be saved in the system dump file. Additionally, if a bugcheck occurs during system initialization, the contents of the register display may be unreliable. The symptom of such a bugcheck is a SHOW SUMMARY display that shows no processes or only the swapper process.

If you use an SDA command to access a virtual address that has no corresponding physical address, SDA generates the following error message:

%SDA-E-NOTINPHYS, 'location': virtual data not in physical memory

When analyzing a selective dump file, if you use an SDA command to access a virtual address that has a corresponding physical address not saved in the dump file, SDA generates one of the following error messages:

%SDA-E-MEMNOTSVD, memory not saved in the dump file

%SDA-E-NOREAD, unable to access location n