If you are using coupling facility structures for
your log streams, you must specify in the CFRM policy the size of
each structure you define. To simplify the task of calculating the
size, IBM® provides the CF Structure
Sizer (CFSizer), a tool that estimates the storage size for each structure
by asking questions based on your existing or planned configuration.
The CF Structure Sizer (CFSizer) uses the selected structure input
to calculate the SIZE and INITSIZE values for the CFRM policy.
The IBM CF
Structure Sizer (CFSizer) can be accessed at http://www.ibm.com/systems/support/z/cfsizer/.
Before using the CF Structure Sizer (CFSizer), you
should go through the following steps for each coupling facility
structure you wish to define. The information that you collect will
be useful both for the CF Structure Sizer (CFSizer) and for defining
your LOGR policy. The procedure will help you determine the minimum
amount of coupling facility space you should allocate for a coupling
facility structure, based on the amount of space needed for all the
log streams mapping to the structure. See Add information about log streams and coupling facility structures to the LOGR policy.
- Gather information about the coupling facility
structure.
For each structure, gather the following information
common for all the log streams that map to the structure.
- AVGBUFSIZE
- Average size of log blocks written by applications to log streams
associated with this coupling facility structure. This value corresponds
to the AVGBUFSIZE parameter specified for each structure in the LOGR
policy. For this procedure, pick the value that represents the average
size of log blocks generated by all the log streams writing to this
coupling facility structure. For specifying the AVGBUFSIZE parameter
in the LOGR policy, see Specifying the average log block size.
For a logrec log stream, IBM recommends an
AVGBUFSIZE of 4068.
For an OPERLOG log stream, IBM recommends an AVGBUFSIZE
of 512.
For an RRS log stream, see z/OS MVS Programming: Resource Recovery.
Installation
or vendor supplied applications should base AVGBUFSIZE on the average
size of the log blocks they want to write to the log stream.
- MAXBUFSIZE
- Maximum size of a log block corresponding to the MAXBUFSIZE
parameter in the LOGR policy.
For a logrec or OPERLOG log stream, IBM recommends a
MAXBUFSIZE of 4096. For an RRS log stream, see z/OS MVS Programming: Resource Recovery.
Installation
or vendor supplied applications should base MAXBUFSIZE on the maximum
size of the log blocks they want to write to the log stream.
Note
that once you have defined the MAXBUFSIZE for a coupling facility
structure, you cannot update the value. To change the MAXBUFSIZE,
you must delete the log streams associated with the structure, delete
the structure, and then re-define the structure with the new MAXGBUFSIZE
value.
- LOGSNUM
- Maximum number of log streams in the structure corresponding
to the LOGSNUM parameter in the LOGR policy. See The LOGSNUM parameter for
more information.
- Gather information about each log stream
For
each log stream that maps to a coupling facility structure, gather
the following information.
Most of these values are projected
or desired values for your log stream, and a few correspond to parameters
you will specify in the LOGR policy.
- LOWOFFLOAD
- LOWOFFLOAD specifies the point, in percent value of space consumed,
where system logger will stop offloading coupling facility log data
to the DASD log data sets for this log stream. It corresponds to the
LOWOFFLOAD parameter in the LOGR policy. For OPERLOG, logrec log stream,
or the CICS® log manager, IBM recommends that
you use the LOWOFFLOAD default value of zero. See Administrative data utility for
more the LOWOFFLOAD parameter.
- HIGHOFFLOAD
- HIGHOFFLOAD specifies the point, in percent value of space consumed,
where system logger will begin offloading coupling facility log data
to the DASD log data sets for this log stream. It corresponds to the
HIGHOFFLOAD parameter in the LOGR policy.
For OPERLOG, logrec log
stream, or the CICS log manager, IBM recommends that
you use the HIGHOFFLOAD default value of 80. See Administrative data utility for
more the HIGHOFFLOAD parameter.
- ResidencyTime
- Desired residency time for log data in the coupling facility
structure, in seconds. Residency time is the amount of time you want
a log block to stay in the coupling facility between the time it is
written to the coupling facility and being off-loaded to DASD.
For
a logrec or OPERLOG log stream, IBM recommends a residency of 10 seconds. For
other system logger applications, get the desired residency time from
the documentation for the application.
- WritePerSec
- Projected write requests per second against the log stream.
This value is the total IXGWRITE requests per second issued by all
system logger applications or systems connected to a log stream.
For an OPERLOG log stream, you can calculate projected
writes per second using the current SYSLOGs. For each system in the
sysplex, do the following calculation:
- Choose a spot in SYSLOG and note the timestamp at that spot. You
might want to choose a peak or high usage time of day.
- Page down 2000 lines by issuing, for example, DOWN 2000 to SDSF
or TSO/E browse.
- Note the time stamp in this spot.
- Calculate the number of seconds between the two time stamps. This
is the number of seconds it took to write 2000 lines of SYSLOG. (This
calculation is based on most messages being one line).
- Divide 2000 by the number of seconds it took to write 2000 lines
to get the lines per second.
- Add the results for each system's SYSLOG to get the total writes
per second for the OPERLOG log stream.
For a logrec log stream, calculate
the projected writes per second using the current logrec data sets.
For each system that will write to the logrec log stream, do the following
calculation:
- Request or obtain an EREP report for a particular time span. It
must be a report that includes all records. For example, take an EREP
daily report that processed all records.
- Near the top of the report, message IFC120I will tell how many
records were written in the time span chosen for the report. Divide
this number by the number of seconds in the time span for the average
writes per second for this logrec data set.
- You can also look at timestamps in the output to analyze how many
records were written in a particular second. You can do this to check
for peak usage.
- Add the results for each logrec data set that will write to the
log stream to get the total writes per second for a logrec log stream.
After generating the estimated structure sizes from the CF Structure
Sizer (CFSizer), if you need to change the coupling facility structure
size later, there are two ways you can do that:
- Dynamically alter the coupling facility structure size by entering
the SETXCF START,ALTER command or issuing the IXLALTER service.
- Update the CFRM policy with the new coupling facility size, activate
the policy, and then have the operator initiate a rebuild of the structure.
See Administrative data utility for updating the CFRM policy using the IXCMIAPU
utility.
Note, however, that these options should not be substituted for
advance planning, which can optimize both capacity and performance.
See the following for more information: