Compressed Dump

Compressed Dump White Paper, Version 1.3
page 3
If compression is turned OFF, then the system will not dump compressed even when the environment of
the crash is suitable to dump compressed.
The compression hint is turned ON by default.
Any changes to the configuration take effect immediately and remain in effect until the next system reboot,
or until changed with a subsequent invocation of crashconf. Using the option -t, changes to the include and
excluse class lists and compression mode can be made persistent across system reboots. But the
changes do not persist across kernel rebuilds. Use SAM or kctune(1M) to do this.
pstat_getcrashinfo(2) can be used to query the current crashdump configuration.
The field psc_flags, will indicate the state of compressed dump.
/*
* This structure describes the system crash dump configuration.
* It is only available as 64-bit data (_PSTAT64 defined).
*/
struct pst_crashinfo {
int64_t psc_flags; /* Dump config. flags, see below */
struct __psdev psc_headerdev; /* Device containing dump
header */
int64_t psc_headeroffset; /* Byte Offset of dump hdr on
device */
int64_t psc_ncrashdevs; /* Number of dump devices */
int64_t psc_totalsize; /* Total amount of dump space (kB) */
int64_t psc_included; /* Page classes to be included */
int64_t psc_excluded; /* Page classes to be excluded */
int64_t psc_default; /* Defaults for unspec’d classes */
int64_t psc_nclasses; /* Number of classes */
int64_t psc_pgcount[PST_MAXCLASSES]; /* Number of pages
in each class */
int64_t psc_reserved; /* Reserved for future use */
};
/* Flag values for psc_flags: */
#define PS_EARLY_DUMP 0x1 /* An early dump was taken */
#define PS_CONF_CHANGED 0x2 /* Config. changed since boot */
#define PS_HEADER_VALID 0x4 /* headerdev and headeroffset valid */
#define PS_COMPRESS 0x8 /* Compressed dump state */
At the time of a system crash, the dump sub-system examines the state of the system and its resources to
determine whether it is possible to use compression. For masking the time taken for compression, addi-
tional processors are required to do compression in parallel with I/O. Also, since pages are dumped com-
pressed, additional data should be maintained in the dump to check for the validity of the dump. These
include checksum and information required to uncompress the pages. If the dump device has adequate
space to hold the compressed dump and processors required to do compression to match I/O speeds are
available, then the system will proceed with compressed dump.
After a system crash, the choice for compression can be changed using the console just like the choice for
full/selective dump.
When the system is dumping compressed, dump progress will be updated on the console at least once
every fifteen seconds.