System logger latch conventions

System logger uses GRS latches to serialize different operations and control resource access. You can use commands from an MVS™ console to identify what logger latches are held. Use the following commands to display which latches are held by logger. See Relevant MVS system commands for more details.

D GRS, Latch, Jobname=IXGLOGR
D GRS,ANALYZE,LATCH,DEPENDENCY,DETAIL
D GRS,C,LATCH

Logger uses three distinct types of latches: miscellaneous latches, structure or task latches, and log stream latches. Table 1 shows the latch set names and latch numbers of the three types of latches.

Table 1. Latch types used by Logger
Latch type	Latch set name	Latch number
Miscellaneous latches	'SYS.IXGLOGER_MISC	8
Structure or task latches	'SYS.IXGLOGER_STRUCTURE_LATCH_SET	1024
Log stream latches	'SYS.IXGLOGER_LCBIT___CTA:12345678_SLSA:1234	224 per latch set name

Logger uses different procedures to create and manage these latches:

Miscellaneous latches:
- Miscellaneous latches are established when the IXGLOGR address space is initialized.
- The latch set name contains six latch numbers.
- Each latch number controls a different logger resource. For example, miscellaneous latch number 7 controls access to the system logger configuration (for example: IXGCNFXX parmlib) settings.
Structure or task latches:
- Structure or task latches are established when the IXGLOGR address space is initialized.
- The latch set name contains 1024 latch numbers.
- There are two sets of latch numbers.
- Each latch number corresponds to a logger connecting task and its corresponding CTA entry.
Log stream latches:
- Log stream latches are established as needed during a log stream connection. When a log stream is associated with a logger connection task, the log is included in the first available latch set name, if any are already established under that connection task.
- Each log stream latch set name is created using the following model:
```
SYS.IXGLOGER_LCBIT___CTA:12345678_SLSA:1234
```
  In the model, each log stream latch set name is uniquely identified by a combination of the CTA number and the SLSA number.
- The CTA number identifies which logger connection task holds the latch.
- The SLSA number identifies a unique latch set instance, within the CTA number, for a log stream. For structure-based log streams, there can be up to 16 latch sets per CTA number. For DASDONLY-based log streams, there can be up to two latch sets per CTA number.
- Each log stream latch set contains 224 latch numbers.
- Each log stream has seven latch subtypes.
- Each latch subtype is represented in a range of indexes within the latch set.

Logger associates a log stream name with a latch number for a log stream latch set. When you use the MVS command D GRS,ANALYZE,LATCH,DEPENDENCY,DETAIL to show global latch contention, use the log stream names to determine the resources that are involved with the latching. The following examples show the outputs by using the D GRS,ANALYZE,LATCH,DEPENDENCY,DETAIL command.

Figure 1 shows output of ISG374I messages from the D GRS,ANALYZE,LATCH,DEPENDENCY,DETAIL command. In this example, system logger (jobname IXGLOGR) is waiting to get the latch exclusive but another job (WRITE3) holds the latch shared.

CTA number 3 identifies the third logger task of the structure connection.
SLSA number 1 is for the second latch set within the logger connecting task 3.
Latch number 2 is the latch number shared for the log stream named IXGLOGR.SOME.LOG.STREAM.

Figure 1. Example: Logger waiting to get latch exclusive

SY1  d grs,an,latch,depend,detail
SY1  ISG374I 14.23.53 GRS ANALYSIS 460
DEPENDENCY ANALYSIS: ENTIRE SYSTEM
----- LONG WAITER #1
         JOBNAME: IXGLOGR   (ASID=002B, TCB=005DDE88)
         REQUEST: EXCLUSIVE                        LT:7F42907800000001
WAITING  00:01:33 FOR RESOURCE (CREATOR ASID=002B)
SYS.IXGLOGER_LCBIT___CTA:00000003_SLSA:0001       LST:7F42BD00000000BD
2:IXGLOGR.SOME.LOG.STREAM
         JOBNAME: WRITE3    (ASID=0026, TCB=005D3A08)
         REQUEST: SHARED                           LT:7F42901000000002
ANALYSIS ENDED: THIS UNIT OF WORK IS NOT WAITING

Figure 2 shows the output of ISG374I messages from the D GRS,ANALYZE,LATCH,DEPENDENCY command. Logger recommends the detail display because the regular display truncates a portion of the latch set name.

Figure 2. Example: Output from ISG374I messages - regular display

SY1  d grs,an,latch,depend
SY1  ISG374I 14.23.48 GRS ANALYSIS 457
DEPENDENCY ANALYSIS: ENTIRE SYSTEM
----- LONG WAITER #1
WAITTIME  JOBNAME  E/S  CASID LSETNAME/LATCHID
00:01:28  IXGLOGR  *E*  002B  SYS.IXGLOGER_LCBIT___CTA:00000003_SLSA T                              
                              2:IXGLOGR.SOME.LOG.STREAM							  
                                                          BLOCKER   WRITE3    S
ANALYSIS ENDED: THIS UNIT OF WORK IS NOT WAITING

Figure 3 shows the output of other ISG374I messages from D GRS,ANALYZE,LATCH,DEPENDENCY,DETAIL command. In this example, system logger is waiting for dasdonly log stream IXGLOGR.DASDONLY.STREAM to get exclusive control of latch 1 in latch set SYS.IXGLOGER_LCBIT___CTA:00000324_SLSA:0001 while job READLOG holds the latch shared.

Figure 3. Example: Output from ISG374I messages

SY1  d grs,an,latch,dependency,detail
SY1  ISG374I 12.55.00 GRS ANALYSIS 553
DEPENDENCY ANALYSIS: ENTIRE SYSTEM
----- LONG WAITER #1
         JOBNAME: IXGLOGR   (ASID=0015, TCB=005DE408)
         REQUEST: EXCLUSIVE                        LT:7F33A01000000000
WAITING  00:00:05 FOR RESOURCE (CREATOR ASID=0015)
SYS.IXGLOGER_LCBIT___CTA:00000324_SLSA:0001       LST:7F33CD000000009D
1:IXGLOGR.DASDONLY.STREAM
         JOBNAME: READLOG   (ASID=002A, TCB=005D7D90)
         REQUEST: SHARED                           LT:7F33B01000000000
ANALYSIS ENDED: THIS UNIT OF WORK IS NOT WAITING