Monitoring log stream interim storage consumption

System logger internally tracks the usage of the log stream interim storage medium. Starting at z/OS® V2R1, key information can be revealed to both the installation and exploiter, depending on the configuration of IXGCNF and log stream parameters.

This information can assist in exposing offload inhibitors, including those inhibitors which may not trigger logger offload or task monitoring alerts – such an abnormal DASD I/O delays.

There are different sets of system logger parameters that affect the level of monitoring for a log stream on a given z/OS system.

The first set of parameters to configure are in the log stream definition in the LOGR policy. They are the WARNPRIMARY and HIGHOFFLOAD parameters. System logger establishes an imminent threshold for each log stream based on an interim storage usage percentage above HIGHOFFLOAD. The imminent threshold value is 2/3rd of the difference between 100% full and the HIGHOFFLOAD value. The WARNPRIMARY parameter indicates whether warning messages for this log stream should be issued when the imminent threshold is reached or a 90% entry full condition is encountered for structure based log streams. The default WARNPRIMARY value is NO.

These parameters have the following effects:

When the imminent threshold is reached, regardless of the WARNPRIMARY parameter, the IXGWRITE ANSAA field ANSAA_WRITEIMMINENTCAPACITY will be set on. See z/OS MVS Programming: Assembler Services Guide. for more details.
When the imminent threshold is reached and WARNPRIMARY(YES) has been specified, message IXG317E will be issued to the console to alert the installation that the primary storage consumption has reached the threshold.
When a 90% structure entry full condition is encountered and WARNPRIMARY(YES) has been specified, message IXG316E will be issued to the console to alert the installation that the primary storage consumption has reached the entry near full threshold.
Note: This condition only occurs for log streams defined to use a CF structure.
When an interim storage full condition is encountered and WARNPRIMARY(YES) has been specified or when a 100% structure entry full condition is encountered, message IXG318E will be issued indicating the interim storage resource is full.

The system logger alert messages remain in the retrievable messages list until the appropriate circumstances occur for each:

The IXG318E constraint condition will be deleted when one of the following occurs:
- After a log stream offload activity, system logger detects the log stream primary (interim) storage consumption appears to be at least 5% less than full, then message IXG319I will be issued and the system will delete message IXG318E.
- When there are no longer any log stream connections on the system.
The IXG317E imminent threshold message will be deleted when one of the following occurs:
- After a log stream offload activity, system logger detects the percent in use falls at least 5% below the imminent threshold established for the log stream.
- When the condition is superseded by a primary (interim) storage full condition and message IXG318E is issued for the log stream.
- When there are no longer any log stream connections on the system.
The IXG316E entry near full threshold message will be deleted when one of the following occurs:
- After a log stream offload activity, system logger detects the percent in use falls at least 85% or lower for the entire CF structure.
- When the condition is superseded by a primary (interim) storage full condition and message IXG318E is issued for any log stream in the CF structure.
- When there are no longer any log stream connections on the system using the CF structure.

The second set of parameters allows these messages to be enabled or disabled on a z/OS system basis. This is done through the IXGCNF CONSUMPTIONALERT parameter. The default setting for this parameter is ALLOW, and the interim storage usage alert messages will be issued at the previously described points. When the parameter is set to SUPPRESS, it indicates that no interim storage warning messages will be issued, regardless of the log stream parameters.

Additionally, XES provides structure utilization monitoring which can be valuable for log streams defined to use CF structures. The FULLTHRESHOLD parameter for a CF structure specification in the CFRM policy can be used to provide consumption warnings via z/OS XES messages (that is IXC585E and IXC586I). Message IXC360I also contains significant CF structure use and consumption information for a DISPLAY XCF,STR,STRNAME=structurename command. For more information, see Monitoring structure utilization.

Note: These settings are critical in cases where the log stream and related resources may be configured to try to avoid physically moving log data during the offload process - that is, the offload would only delete data from interim storage and not move it to DASD. This configuration is an approach to avoiding the most common offload inhibitors which involve the obtaining of DASD resources. For more information on this configuration, see Managing log data in interim storage to minimize movement to DASD

When interim storage consumption reaches a critical point, look for indications that offload processing is not running in a normal manner:

Review the complete description of any threshold message issued, such as IXG316E, IXG317E or IXG318E.
Look for offload and service task monitoring related messages, such as IXG271I, IXG272E, IXG311I or IXG312E. See Offload and service task monitoring.

For more information, see: