Table of Contents
Overview : gcs log flush sync wait event
- Before sending a reconstructed CR block or CUR block, LMS will verify that corresponding redo vectors are flushed to disk.
- LMS will flush request a log flush when there are uncomitted transactions within the required block -> Increasing commit frequence will increase this wait event too
- Use lfsdiag.sql script to understand the wait time suffered by LGWR process
- If the redo vector are not flushed, LMS on node2 need to wait for ‘gcs log flush sync’ event after requesting LGWR for a log flush, analogous to ‘log file sync’ event.
- in a healthy database 90 % of GCS Log flush Sync should be 2ms ore less
- Always check related instances for LOG FILE SYNC event as this event also reduces the Redo I/O bandwith
- If gcs log flush sync wait event is significant then you need to understand your LGWR performance ( you may use lfsdiag,sql script )
Monitor gv$session wait events for LGWR, LMS processes
SQL> select w.inst_id, w.sid,w.program, w.event , w.STATE , w.SECONDS_IN_WAIT SEC_in_WAIT , w.P1TEXT || ': ' || w.P1 || ' - ' || w.P2TEXT || ': ' || w.P2 || ' - '
|| w.P3TEXT || ': ' || w.P3 as P1_P2_P3 from gv$session w where
program like '%LGWR%' or program like '%LMS%' or program like '%JDBC%' order by program;
INST_ID SID PROGRAM EVENT STATE SEC_IN_WAIT P1_P2_P3
------- ---------- -------------------------------- ---------------------------- ------------------ ----------- ---------------------------------------------
2 18 oracle@grac42.example.com (LGWR) log file parallel write WAITED SHORT TIME 0 files: 1 - blocks: 1 - requests: 1
2 13 oracle@grac42.example.com (LMS0) gcs log flush sync WAITING 0 waittime: 30 - poll: 0 - event: 138
Gcs Log Flush Sync Wait histogramms
Script: @event_histogram_from_awr.sql Acceptable gcs log flush sync histogramm (90 % of wait requests returns in less that 2 ms ) BEGIN_INTERVAL_TIME Inst SNAP_ID EVENT_NAME WAIT_TIME_MILLI WAIT_COUNT ------------------------------ ---- ---------- ---------------- --------------- ---------- 28-NOV-13 04.24.27.507 PM 1 876 gcs log flush sync 1 58940 1 876 gcs log flush sync 2 51775 1 876 gcs log flush sync 4 1433 1 876 gcs log flush sync 8 605 1 876 gcs log flush sync 16 271 1 876 gcs log flush sync 32 181 1 876 gcs log flush sync 64 133 1 876 gcs log flush sync 128 78 1 876 gcs log flush sync 256 7 1 876 gcs log flush sync 512 1 Gcs log flush sync wait histogramm with high wait times 28-NOV-13 04.25.58.021 PM 1 877 gcs log flush sync 1 304 1 877 gcs log flush sync 2 662 1 877 gcs log flush sync 4 11816 1 877 gcs log flush sync 8 46981 1 877 gcs log flush sync 16 68740 1 877 gcs log flush sync 32 202 1 877 gcs log flush sync 64 141 1 877 gcs log flush sync 128 83 1 877 gcs log flush sync 256 8 1 877 gcs log flush sync 512 1