Log Activity

This topic shows detailed information about Statistics - Log Activity.

Statistics - Log Activity

The field labels shown in the following sample layout of Statistics - Log Activity are described in the following section.


LOG ACTIVITY                 QUANTITY  /SECOND  /THREAD  /COMMIT
---------------------------  --------  -------  -------  -------
READS SATISFIED-OUTPUT BUFF     96.00     0.53      N/C     2.91
READS SATISFIED-OUTP.BUF(%)    100.00
READS SATISFIED-ACTIVE LOG       0.00     0.00      N/C     0.00
READS SATISFIED-ACTV.LOG(%)      0.00
READS SATISFIED-ARCHIVE LOG      0.00     0.00      N/C     0.00
READS SATISFIED-ARCH.LOG(%)      0.00

TAPE VOLUME CONTENTION WAIT      0.00     0.00      N/C     0.00
READ DELAYED-UNAVAIL.RESOUR      0.00     0.00      N/C     0.00
ARCHIVE LOG READ ALLOCATION      0.00     0.00      N/C     0.00
ARCHIVE LOG WRITE ALLOCAT.       0.00     0.00      N/C     0.00
CONTR.INTERV.OFFLOADED-ARCH      0.00     0.00      N/C     0.00
LOOK-AHEAD MOUNT ATTEMPTED       0.00     0.00      N/C     0.00
LOOK-AHEAD MOUNT SUCCESSFUL      0.00     0.00      N/C     0.00

UNAVAILABLE OUTPUT LOG BUFF      0.00     0.00      N/C     0.00
OUTPUT LOG BUFFER PAGED IN       0.00     0.00      N/C     0.00

LOG RECORDS CREATED          19276.00   106.05      N/C   584.12
LOG CI CREATED                 861.00     4.74      N/C    26.09
LOG WRITE I/O REQ (LOG1&2)     554.00     3.05      N/C    16.79
LOG CI WRITTEN (LOG1&2)       2004.00    11.02      N/C    60.73
LOG RATE FOR 1 LOG (MB)           N/A     0.02      N/A      N/A

READS SATISFIED-OUTPUT BUFF

The number of log reads satisfied from the output buffer.

Background and Tuning Information

This field, together with the reads satisfied from active log and reads satisfied from archive log (QJSTRACT and QJSTRARH) fields indicate how efficiently Db2 retrieves log records. Use these numbers to adjust the number of output buffers and the total active log capacity to maximize Db2 performance.

Field Name: QJSTRBUF

This is an exception field.

READS SATISFIED-OUTP.BUF(%)

The percentage of log reads that were satisfied in the output log buffer.

Field Name: SARLRBUF

READS SATISFIED-ACTIVE LOG

The number of log reads satisfied from the active log data set.

Background and Tuning Information

This field, together with the reads satisfied from archive log and reads satisfied from output buffer fields, indicate how efficiently Db2 retrieves log records. Use these numbers to adjust the number of output buffers and the total active log capacity to maximize Db2 performance. Ideally, this value should be 0 or very small.

Field Name: QJSTRACT

This is an exception field.

READS SATISFIED-ACTV.LOG(%)

The percentage of log reads satisfied from the active log.

Field Name: SARLRACT

This is an exception field.

READS SATISFIED-ARCHIVE LOG

The number of log reads satisfied from the archive log data set.

Background and Tuning Information

This field, together with the reads satisfied from active log and reads satisfied from output buffer fields indicate how efficiently Db2 retrieves log records. Use these numbers to adjust the number of output buffers and the total active log capacity to maximize Db2 performance. Ideally, this value should be 0 or very small.

Field Name: QJSTRARH

This is an exception field.

READS SATISFIED-ARCH.LOG(%)

The percentage of log reads that were satisfied from the archive log data set.

Field Name: SARLRARC

This is an exception field.

TAPE VOLUME CONTENTION WAIT

The number of read accesses that were delayed because of a tape volume contention when only one reader per tape is possible.

Background and Tuning Information

This field shows the number of agents forced to wait because a tape volume was already in use by another. If this number is not 0, increase the read tape units on the archive log data set parameters panel DSNTIPA.

Field Name: QJSTTVC

This is an exception field.

READ DELAYED-UNAVAIL.RESOUR

The number of read accesses delayed due to unavailable resources.

Background and Tuning Information

Generally, this can be due to insufficient tape units allocated. If this is so, reissue the SET ARCHIVE command and use a higher value for the count parameter. Another (although unlikely) cause is insufficient archive log read service task availability.

Field Name: QJSTWUR

This is an exception field.

ARCHIVE LOG READ ALLOCATION

The number of archive log read allocations.

It indicates the frequency of archive log open and close activity.

Background and Tuning Information

A high number indicates a need for more or larger active log data sets. This value should be small, ideally 0.

Field Name: QJSTALR

This is an exception field.

ARCHIVE LOG WRITE ALLOCAT.

The number of archive log write allocations.

It indicates the frequency of archive log open and close activity.

Background and Tuning Information

A high number indicates a need for more or larger active log data sets. This value should be small, ideally 0.

Field Name: QJSTALW

CONTR.INTERV.OFFLOADED-ARCH

The number of control intervals (CIs) offloaded from the active log to the archive log.

Field Name: QJSTCIOF

LOOK-AHEAD MOUNT ATTEMPTED

The number of look ahead (tape volume) mounts attempted.

Background and Tuning Information

This field and field QJSTLAMS (label LOOK-AHEAD MOUNT SUCCESSFUL) show the efficiency of look ahead for tape mounts.

Field Name: QJSTLAMA

LOOK-AHEAD MOUNT SUCCESSFUL

The number of successful look-ahead (tape volume) mounts. It indicates the look-ahead mounting performance gains.

Background and Tuning Information

For maximum performance, this field and field QJSTLAMA (label LOOK-AHEAD MOUNT ATTEMPTED) should be equal. To find the number of failed attempts, subtract the value in this field from LOOK-AHEAD MOUNT ATTEMPTED. Too many failed attempts negate potential performance gains. This can be caused by not having enough tape units available. Issue the DISPLAY ARCHIVE command and note the current count value. Then issue the SET ARCHIVE command using a higher value for the count parameter.

Field Name: QJSTLAMS

UNAVAILABLE OUTPUT LOG BUFF

The number of waits caused by an unavailable output log buffer.

When Db2 wants to write a log record and the log buffer is not available, Db2 and the application must wait for an available log buffer.

Background and Tuning Information

Another possible cause is that the size of the write threshold might be too close to the size of the output buffer.

If this field is not 0, increase the number in the output buffer field on installation panel DSNTIPL to increase the number of output buffers or increase the size of the buffer.

Field Name: QJSTWTB

This is an exception field.

OUTPUT LOG BUFFER PAGED IN

The number of times an output log buffer had to be paged in before it could be initialized. The log-write latch is held at this point.

Background and Tuning Information

A nonzero value could indicate that the output log buffer size is too large, or there is insufficient real storage to back up the output log buffer size.

Field Name: QJSTBPAG

LOG RECORDS CREATED

The number of log write requests.

The log record is written asynchronously to the log buffer. The application does not wait for the record to be written to the log data set and regains control immediately.

Buffered log records are written to DASD when the buffer threshold is exceeded.

Field Name: QJSTWRNW

LOG CI CREATED

The number of active log output control intervals created.

Background and Tuning Information

Log records are placed sequentially in output log buffers, which are formatted as VSAM control intervals. The control intervals are written to a set of predefined DASD active log data sets, which are used sequentially and recycled.

The ratio of this field to write output log buffers should be low.

Rules of thumb:

The lower the value, the better. A high value indicates that too many I/Os are required for the number of log buffers created.

It is possible that WRTTHRSH is set too low. It is also possible that transactions could be arriving so infrequently that at commit time force requests are not queued and each force request is individually triggering an I/O of its log buffers.

Field Name: QJSTBFFL

LOG WRITE I/O REQ (LOG1&2)

The total number of log-write I/O requests (such as media manager calls). This is the sum of the IFCID 038/039 pairs and includes both copy1 and copy2 active log data set writes.

Background and Tuning Information

This value should correspond to the active log write I/O activity in an RMF report.

Field Name: QJSTLOGW

LOG CI WRITTEN (LOG1&2)

The total number of log control intervals (CIs) written. This includes CI rewrites and both copy1 and copy2 active log data set writes. If a given CI is rewritten 5 times, this counter is incremented by 5.

Field Name: QJSTCIWR

LOG RATE FOR 1 LOG (MB)

The log rate for the active log data sets in MB per second. This figure is valid for dual logging, if single logging is used, multiply the value shown by 2.

Background and Tuning Information

To calculate this rate (mega bytes/second) at which data is written to the active log data set, multiply the value of field QJSTCIWR (label LOG CI WRITTEN (LOG1&2)) by 4096 and divide it by 1024 * 1024 * statistics-interval-seconds * 2. When the value exceeds 10MB/sec per log copy, you should examine I/O tuning of log data sets (for example, using faster log devices and/or I/O striping, using variable-length or compressed log record layouts to reduce log data size).

Field Name: SJSTCIWR