Logical standby: SQL Apply too slow

Question

Hi all,
I have a question regarding SQL Apply performance in logical standby. There are two kind of operations that are remarkably slow when applying them on logical standby. These are "truncate table" and "delete from table" operations.
When logical standby pick up one of mentioned statements from logs one of appliers start working whereas rest others are waiting. It looks like standby hang and very slow sql apply is moving on gradually and finally when operation completes standby is behind primary for 4 or 5 or even 8 hours.
What can be done in this regard to speed up sql apply and alleviate this situation?
Best Regards,
Alex

Answer 1

Are you absolutely sure that the truncate is the problem (and deletes). How did you check it?
You can use LogMiner to check what are most of the commands in the log currently applied. I use this:
BEGIN
sys.DBMS_LOGMNR.ADD_LOGFILE( LOGFILENAME => '/home/oracle/arc_43547_1_595785865.arc', OPTIONS => sys.DBMS_LOGMNR.ADDFILE);
END;
BEGIN
sys.DBMS_LOGMNR.START_LOGMNR( OPTIONS => sys.DBMS_LOGMNR.DICT_FROM_ONLINE_CATALOG);
END;
SELECT seg_owner,seg_name,table_name,operation,COUNT(1) FROM V$LOGMNR_CONTENTS
GROUP BY seg_owner,seg_name,table_name,operation
ORDER BY COUNT(1) DESC
BEGIN
sys.DBMS_LOGMNR.END_LOGMNR();
END;
Most of the times in our cases when SQL Apply is slow is because of high activity on particular object. This can be detected by high number of DMLs for that object using LogMiner. If this object is not needed on the logical standby you can skip it and thus SQL Apply will be faster because it will not apply changes for this particular one. If it's needed and this is not a regular rate, then you can skip it temporarily, turn on SQL Apply , after problematic logs are applied, turn off SQL Apply, instantiate the object and unskip it, turn on sql apply again.
Another thing that can drastically slowdown SQL Apply is the size of memory available for SQL Apply(Alert log shows that max is ~4.5GB or something like this, I'm not sure )
You can increase it with something like this:
ALTER DATABASE STOP LOGICAL STANDBY APPLY;
BEGIN
DBMS_LOGSTDBY.APPLY_SET('MAX_SGA', 3000); -- set to 3000 MB
END;
ALTER DATABASE START LOGICAL STANDBY APPLY;
You have to increase it if the following reports:
SELECT NAME, VALUE FROM V$LOGSTDBY_STATS
WHERE NAME LIKE '%page%' OR
NAME LIKE '%uptime%' or name like '%idle%';
that 'bytes paged out' increases if run every few seconds during slow SQL Apply.
I hope that it's something that can be fixed using the above info. If no, please comment and share your investigations.
Thanks