Host=rac1.example.com Target type=Cluster Target name=myrac Message=DB Checkpoint Performance on Host rac1 Database/Cluster racdba Instance racdba1. The Cluster Health Advisor (CHA) detected that Database Writer (DBW) checkpoints were slow because the database writes took longer than expected to complete. Increase the number of DBWR processes. Add additional disks to the disk group for the database. Relocate the database files to faster disks or to Solid State Devices. If the storage subsystem supports a storage write back cache, check that the storage cache is functioning properly. Severity=Warning Event reported time=Feb 23, 2021 12:04:53 PM GMT Operating System=Linux Platform=x86_64 Event Type=Metric Alert Event name=cha_alerts:cha_alert_level Metric Group=CHA Alerts Metric=Alert Level Metric value=Warning Key Value=CHA_INCIDENT_STATE_CHANGE_DATABASE_racdba_racdba1_CHA_DBWR_CKPT_IO_COMPLETE Key Column 1=Key Rule Name=SYSMAN Monitor Targets,rule 61 Rule Owner=SYSMAN Update Details: DB Checkpoint Performance on Host rac1 Database/Cluster racdba Instance racdba1. The Cluster Health Advisor (CHA) detected that Database Writer (DBW) checkpoints were slow because the database writes took longer than expected to complete. Increase the number of DBWR processes. Add additional disks to the disk group for the database. Relocate the database files to faster disks or to Solid State Devices. If the storage subsystem supports a storage write back cache, check that the storage cache is functioning properly.I observed some infamous checkpoint not complete messages in the alert log, so I suspected that they might be the real cause.
Thus, I created quite small log files 20MB in size:
SQL> select group#, thread#, bytes from v$log;
GROUP# THREAD# BYTES
---------- ---------- ----------
5 1 20971520
6 1 20971520
7 1 20971520
8 2 20971520
9 2 20971520
10 2 20971520
6 rows selected.
Then, I wrote a script that generates a lot of redo:
create table t
as
select o.*
from dba_objects o,
xmltable('1 to 50');
update t
set object_name=object_name;
I ran the script and here is an excerpt from the alert log showing checkpoint not complete messages:
2021-02-23T12:04:47.234452+00:00 Thread 1 advanced to log sequence 114 (LGWR switch), current SCN: 7677512 Current log# 6 seq# 114 mem# 0: +DATA/RACDBA/ONLINELOG/group_6.290.1065267481 2021-02-23T12:04:47.506174+00:00 PDB(3):Resize operation completed for file# 12, old size 645120K, new size 650240K 2021-02-23T12:04:47.848002+00:00 Thread 1 cannot allocate new log, sequence 115 Checkpoint not complete Current log# 6 seq# 114 mem# 0: +DATA/RACDBA/ONLINELOG/group_6.290.1065267481 2021-02-23T12:04:50.392351+00:00 Thread 1 advanced to log sequence 115 (LGWR switch), current SCN: 7677540 Current log# 7 seq# 115 mem# 0: +DATA/RACDBA/ONLINELOG/group_7.291.1065267483 2021-02-23T12:04:50.661220+00:00 PDB(3):Resize operation completed for file# 12, old size 650240K, new size 655360K 2021-02-23T12:04:50.993950+00:00 Thread 1 cannot allocate new log, sequence 116 Checkpoint not complete Current log# 7 seq# 115 mem# 0: +DATA/RACDBA/ONLINELOG/group_7.291.1065267483 2021-02-23T12:04:53.576282+00:00 Thread 1 advanced to log sequence 116 (LGWR switch), current SCN: 7677574 Current log# 5 seq# 116 mem# 0: +DATA/RACDBA/ONLINELOG/group_5.289.1065267477 2021-02-23T12:04:53.845381+00:00 PDB(3):Resize operation completed for file# 12, old size 655360K, new size 660480K 2021-02-23T12:04:54.183174+00:00 Thread 1 cannot allocate new log, sequence 117 Checkpoint not complete Current log# 5 seq# 116 mem# 0: +DATA/RACDBA/ONLINELOG/group_5.289.1065267477 2021-02-23T12:04:56.721595+00:00 Thread 1 advanced to log sequence 117 (LGWR switch), current SCN: 7677600 Current log# 6 seq# 117 mem# 0: +DATA/RACDBA/ONLINELOG/group_6.290.1065267481 2021-02-23T12:04:56.995439+00:00 PDB(3):Resize operation completed for file# 12, old size 660480K, new size 665600K 2021-02-23T12:04:57.332102+00:00 Thread 1 cannot allocate new log, sequence 118 Checkpoint not complete Current log# 6 seq# 117 mem# 0: +DATA/RACDBA/ONLINELOG/group_6.290.1065267481Shortly afterwards, I got a new CHA alert in the OEM:
To understand what is happening, firstly, I started looking for some clues in the CHA log:
/u01/app/grid/crsdata/rac1/trace/chad/ochad.trc.0, in which /u01/app/grid is ORACLE_BASE of the grid user.
[Thread-22] [ 2021-02-23 12:04:53.390 UTC ] [BNet.validate:422] INFO: Tue Feb 23 12:04:50 UTC 2021 Decided CHA_DBWR_CKPT_IO_COMPLETE NORMAL (Belief= 94.900925 %) --> ABNORMAL (Belief= 81.315659 %)
[Thread-22] [ 2021-02-23 12:04:53.390 UTC ] [BNet.validate:429] INFO: Tue Feb 23 12:04:50 UTC 2021 Children
[Thread-22] [ 2021-02-23 12:04:53.391 UTC ] [BNet.validate:446] INFO: Tue Feb 23 12:04:50 UTC 2021 DBWR IO Completion= NORMAL
[Thread-22] [ 2021-02-23 12:04:53.391 UTC ] [BNet.validate:446] INFO: Tue Feb 23 12:04:50 UTC 2021 log_file_switch_checkpoint_incompleteRANGE3= HIGH
[Thread-22] [ 2021-02-23 12:04:53.391 UTC ] [EventPublisher$EventRequest.dump:193] INFO: type:DATABASE, target:racdba, host:rac1, instance:racdba1,
problem:CHA_DBWR_CKPT_IO_COMPLETE, state:ABNORMAL, time:2021-02-23 12:04:50,
evidence:[{"signal":"log_file_switch_checkpoint_incomplete", "fd":"log_file_switch_checkpoint_incompleteRANGE3", "state":"HIGH", "value":1.09745E06, "pred":null, "since":"2021-02-23 12:04:50"}]
Based on the evidence above, I can conclude that CHA is checking the log file switch (checkpoint incomplete) wait event:
SQL> select event, total_waits, total_timeouts, time_waited_micro 2 from v$system_event 3 where event='log file switch (checkpoint incomplete)' 4 / EVENT TOTAL_WAITS TOTAL_TIMEOUTS TIME_WAITED_MICRO ---------------------------------------- ----------- -------------- ----------------- log file switch (checkpoint incomplete) 166 0 196362926The following process accesses this file:
[grid@rac1 grid]$ fuser /u01/app/grid/crsdata/rac1/trace/chad/ochad.trc.0 /u01/app/grid/crsdata/rac1/trace/chad/ochad.trc.0: 13858 [grid@rac1 grid]$ ps fww -p 13858 PID TTY STAT TIME COMMAND 13858 ? Sl 0:37 /u01/app/19.3.0/grid/jdk/bin/java -server -Xms30M -Xmx512M -Djava.awt.headless=true -Ddisable.checkForUpdate=true -DTRACING.ENABLED=false -XX:ParallelGCThreads=1 -cp /u01/app/19.3.0/grid/jlib/cha.jar:/u01/app/19.3.0/grid/jlib/chaconfig.jar:/u01/app/19.3.0/grid/jlib/cha-diag-msg.jar:/u01/app/19.3.0/grid/jlib/clsce.jar:/u01/app/19.3.0/grid/jlib/srvm.jar:/u01/app/19.3.0/grid/jlib/srvmhas.jar:/u01/app/19.3.0/grid/jlib/srvmasm.jar:/u01/app/19.3.0/grid/jlib/netcfg.jar:/u01/app/19.3.0/grid/jdbc/lib/ojdbc8.jar:/u01/app/19.3.0/grid/ucp/lib/ucp.jar:/u01/app/19.3.0/grid/jlib/fte.jar:/u01/app/19.3.0/grid/jlib/jai_core.jar:/u01/app/19.3.0/grid/jlib/mlibwrapper_jai.jar:/u01/app/19.3.0/grid/jlib/vecmath.jar:/u01/app/19.3.0/grid/jlib/jai_codec.jar:/u01/app/19.3.0/grid/jlib/jh.jar oracle.cha.server.CHADDriverThe corresponding CRS resource is ora.chad:
[grid@rac1 trace]$ crsctl stat res ora.chad -t
--------------------------------------------------------------------------------
Name Target State Server State details
--------------------------------------------------------------------------------
Local Resources
--------------------------------------------------------------------------------
ora.chad
ONLINE ONLINE rac1 STABLE
ONLINE ONLINE rac2 STABLE
--------------------------------------------------------------------------------
It can be managed by srvctl commands: srvctl stop/status/start cha.CHA is tightly integrated with the Grid Infrastructure Management Repository (GIMR). In fact, most chactl commands cannot work without it. However, I do not use GIMR, and it appears that some CHA functionality is still available, at least part of it that can publish database related events to OEM. It is explained in the following whitepaper: Oracle Database 12c Rel. 2 Cluster Health Advisor - Deep Dive How it Works and How to Use It
Conclusion
I do not think that the message produced by the CHA_DBWR_CKPT_IO_COMPLETE alert has appropriate solutions:DB Checkpoint Performance on Host rac1 Database/Cluster racdba Instance racdba1. The Cluster Health Advisor (CHA) detected that Database Writer (DBW) checkpoints were slow because the database writes took longer than expected to complete. Increase the number of DBWR processes. Add additional disks to the disk group for the database. Relocate the database files to faster disks or to Solid State Devices. If the storage subsystem supports a storage write back cache, check that the storage cache is functioning properly.It is usually much easier to increase the total redo size, either by adding additional log groups, changing the size of the existing ones, or both. All of these actions give more chances to DBWR to catch up.
When there are CHA alerts in OEM, it makes sense to check the CHA log:
/u01/app/grid/crsdata/rac1/trace/chad/ochad.trc.0. It has some invaluable information that could reveal exactly the cause.
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