Technical data

SDA Description
Using PAGEFILE.SYS
If SYS$SYSTEM:SYSDUMP.DMP does not exist, the operating system writes
the dump of physical memory into SYS$SYSTEM:PAGEFILE.SYS, the system’s
paging 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. If it is clear, the entire paging file is used for paging, and any dump
written to the paging file is lost.
2
Do not use a selective dump (DUMPSTYLE=1) style with PAGEFILE.SYS. If the
PAGEFILE is used for a selective dump, and if the PAGEFILE is not large enough
to contain all the logical memory blocks, the dump fills the entire pagefile and the
system may hang on reboot. When selective dumping is setup, all available space
will be used to write out the logical memory blocks. If the pagefile is large enough
to contain all of physical memory, there is no reason to use selective dumping and
a full memory dump (DUMPSTYLE=0) should be used.
To calculate the minimum size for SYS$SYSTEM:PAGEFILE.SYS, use the
following formula:
size-in-blocks(SYS$SYSTEM:PAGEFILE.SYS)
= size-in-pages(physical-memory)
+ (number-of-error-log-buffers * blocks-per-buffer)
+1
+ 1000
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. (See the OpenVMS
System Manager’s Manual, Volume 1: Essentials and OpenVMS System
Manager’s Manual, Volume 2: Tuning, Monitoring, and Complex Systems
for more information.)
Freeing Space in PAGEFILE.SYS
If you use SYS$SYSTEM:PAGEFILE.SYS to hold system crash dumps, you must
later free the space occupied by the dump so that the pager can use it. Usually,
you include SDA commands in the site-specific startup command procedure
(SYS$MANAGER:SYSTARTUP_VMS.COM) to free this space; if you do not, your
system might hang during the startup procedure.
A common method of freeing space is to copy the dump from
SYS$SYSTEM:PAGEFILE.SYS to another file, using the SDA COPY command.
(Although you can also use the DCL COPY command to copy a dump file, only
the SDA COPY command frees the pages occupied by the dump from the system’s
paging file.)
Occasionally, you might want to free the pages in the paging file that are taken
up by the dump without having to copy the dump elsewhere. When you issue the
ANALYZE/CRASH_DUMP/RELEASE command, SDA immediately releases the
pages to be used for system paging, effectively deleting the dump.
2
The SAVEDUMP parameter is clear by default. To examine or change its value, consult
the OpenVMS System Management Utilities Reference Manual.
SDA–5