The SNAP dump generator bridges the gap between the dump generators SLED and CDUMP. SNAP is part of BS2000 OSD/BC.
During dump generation by CDUMP, the operating system continues to run and may thus “corrupt” the data to be included in the dump. SLED terminates the operating system, so you need a relatively long time to dump the data and restart the system.
SNAP interrupts the operating system for not more than 24 seconds, dumps specific memory areas (see below) and then restarts the operating system. SNAP operates independently of BS2000 and therefore does not corrupt the diagnostic data involved in the dump.
SNAP is called by the operating system from the TPR or SIH status via the $SNAP interface. It is generally called whenever there is an inconsistency in the operating system status that is, however, not serious enough to cause the session to be aborted.
The SNAPTIME system parameter can be used to control when SNAP returns control to BS2000. The default value is 24 seconds. This is the maximum value with SNAP, since the system state “BS2000 terminated” could otherwise occur. Depending on SNAPTIME, the size of the SNAP dump is limited. The maximum dump size depends primarily on the data transfer rate of the I/O and on the speed of the disk containing the file $TSOS.SNAPFILE. With the servers and disks supported by SNAP, the maximum size of a SNAP dump is 1 GB.
A SNAP dump contains the following data:
class 1, class 3 and, optionally, resident class 4 memory (here, resident means that the page is located in main memory)
In BS2000/OSD-BC V9.0 and higher main memory above the 2 GB boundary and thus, possibly, the entire resident class 4 memory is also contained in the SNAP dump.
name list and entry point list of all operating system modules (EOLDTAB)
administration data
hardware status register of the current virtual machine
The SNAP run is logged in the form of messages at the console.
The messages of SNAP have the message class NSP. Information on individual messages can be obtained in ongoing operation with the HELP-MSG-INFORMATION command.
In particular, the SNAP function is also used by the CDUMP dump generator to create a consistent (uncorrupted) backup of class 1, class 3 and resident class 4 memory.