IBM AIX Continuous Availability Features - IBM Redbooks

This means that traditional AIX system trace is the preferred Second Failure Data Capture
(SFDC) tool, because you can more precisely specify the exact trace hooks of interest, given
knowledge gained from the initial failure.
Traditional AIX system trace also provides options that allow you to automatically write the
trace information to a disk-based file (such as /var/adm/ras/trcfile). Lightweight memory trace
provides no such option to automatically write the trace entries to disk when the memory
trace buffer fills. When an LMT memory trace buffer fills, it “wraps”, meaning that the oldest
trace record is overwritten.
The value of LMT derives from being able to view some history of what the system was doing
prior to reaching the point where a failure is detected. As previously mentioned, each CPU
has a memory trace buffer for common events, and a smaller memory trace buffer for rare
events.
The intent is for the “common” buffer to have a 1- to 2-second retention (in other words, have
enough space to record events occurring during the last 1 to 2 seconds, without wrapping).
The “'rare” buffer should have at least an hour's retention. This depends on workload, and on
where developers place trace hook calls in the AIX kernel source and which parameters they
trace.
Disabling and enabling lightweight memory trace
You can disable lightweight memory trace by using the following command:
/usr/bin/raso -r -o mtrc_enabled=0
You can enable lightweight memory trace by using the following command:
/usr/bin/raso -r -o mtrc_enabled=1
Note: In either case, the boot image must be rebuilt (the bosboot command needs to be
run), and the change does not take effect until the next reboot.
Lightweight Memory Trace memory consumption
The default amount of memory required for the memory trace buffers is automatically
calculated based on factors that influence software trace record retention, with the target
being sufficiently large buffers to meet the retention goals previously described.
There are several factors that may reduce the amount of memory automatically used. The
behavior differs slightly between the 32-bit (unix_mp) and 64-bit (unix_64) kernels. For the
64-bit kernel, the default calculation is limited such that no more than 1/128 of system
memory can be used by LMT, and no more than 256 MB by a single processor.
The 32-bit kernel uses the same default memory buffer size calculations, but further restricts
the total memory allocated for LMT (all processors combined) to 16 MB. Table 3-1 presents
some example LMT memory consumption.
Table 3-1 Lightweight Memory Trace memory consumption
Machine Number of
CPUs
POWER3 (375
MHz CPU)
POWER3 (375
MHz CPU)
58 IBM AIX Continuous Availability Features
System memory Total LMT
memory: 64-bit
kernel
1 1 GB 8 MB 8 MB
2 4 GB 16 MB 16 MB
Total LMT
memory: 32-bit
kernel