Oracle Data Guard and Database Incarnations

// Data Guard // Database Incarnations // Standby Logs applying in Alert Log but not in V$ARCHIVED_LOG

Over the past week I’ve been getting a great introduction to the practical workings of Data Guard. In the past I’ve worked a lot with Disaster Recovery systems built using Oracle standard edition and therefore not licensed to use Data Guard and in those circumstances a poor man’s version of the system was put in place by using RSYNC to synchronise archive logs between a production database and a DR database. With the logs in place, a script run on a schedule would recover the data from the logs. It turns out that the concept of Data Guard is pretty similar in that it’s basically about getting archive logs to the right place and setting the destination database into a mode where it can read those logs and be ready for when disaster strikes.

Using Data Guard in a production setting makes for interesting times as you get into the hows and whys of setting up the Data Guard source and destinations and then on into considerations like physical vs. logical DR databases, as well as the debate about the correct terms to use (e.g. is the DR database an “active standby” or “active data guard” system?).

Over the past couple of days I’ve been testing a procedure for switching between the production system and the DR system. The environment is made up of two geographic locations (actually situated in two different cities, which is just like something from the official documentation). On both sites there are two RAC nodes, but only on the production site are both nodes up and running – the standby is open (in read only) as a single instance, or more accurately, one node of a two-node RAC.

The procedure for the switch over is straight forward enough but during testing I did encounter something unusual with one of the scripts used to ensure archive logs are being applied to the standby. Here’s the query that caused the problem:

SELECT SEQUENCE#, TO_CHAR(FIRST_TIME, 'DD-MON-YY HH24:MI:SS') FIRST_TIME,
TO_CHAR(NEXT_TIME, 'DD-MON-YY HH24:MI:SS') NEXT_TIME, APPLIED
FROM V$ARCHIVED_LOG WHERE THREAD#=1 ORDER BY SEQUENCE#;

This query returns details of log application coming from thread #1, i.e. the first node of the source RAC – changing the thread number to 2 gets details of the logs from the other node. The end of the output of that query is:

SEQUENCE# FIRST_TIME NEXT_TIME APPLIED
---------- ------------------ ------------------ ---------
48192 08-JUL-13 22:33:16 09-JUL-13 00:30:17 YES
48193 09-JUL-13 00:30:17 09-JUL-13 00:56:27 NO
48193 09-JUL-13 00:30:17 09-JUL-13 00:56:27 NO
48193 09-JUL-13 00:30:17 09-JUL-13 00:56:27 NO
48193 09-JUL-13 00:30:17 09-JUL-13 00:56:27 YES

Never mind the Yes’s and No’s for now and instead focus your attention on the NEXT_TIME date, the 9th July 2013…. Today is the 24th of July! Where are today’s logs? Strangely, the alert log for the standby database is showing that everything is OK and that recovery of logs is proceeding as expected. It turns out that the problem is down to the Incarnation of the database and a misplaced ORDER BY clause.

For more on database incarnations you really should check out the official documentation, particularly the neat diagram that really explains what’s happening when you issue a RESETLOGS command, you can find all that here: http://docs.oracle.com/cd/E11882_01/backup.112/e10642/rcmrvcon.htm#BRADV1171

In my case, the incarnation had moved on (as a result of something we were trying out last week) and so log sequence numbers are now being reused. The query above orders its results by the SEQUENCE# and so the highest sequence number displays last, but those high log sequence numbers were generated by a previous incarnation of the database and so are not current. A simple change to the query reveals the truth of the situation:

SELECT SEQUENCE#, TO_CHAR(FIRST_TIME, 'DD-MON-YY HH24:MI:SS') FIRST_TIME,
TO_CHAR(NEXT_TIME, 'DD-MON-YY HH24:MI:SS') NEXT_TIME, APPLIED
FROM V$ARCHIVED_LOG WHERE THREAD#=1;

With the ORDER BY removed, the results of the query look a lot more reassuring:

SEQUENCE# FIRST_TIME NEXT_TIME APPLIED
---------- ------------------ ------------------ ---------
3208 24-JUL-13 14:11:52 24-JUL-13 14:11:54 YES
3209 24-JUL-13 14:11:54 24-JUL-13 14:11:59 YES
3210 24-JUL-13 14:11:59 24-JUL-13 14:12:02 YES
3211 24-JUL-13 14:12:02 24-JUL-13 14:13:57 YES
3212 24-JUL-13 14:13:57 24-JUL-13 14:14:50 YES

Note the correct date and the low, low log sequence numbers!

Keeping track of the time, so to speak, when the database incarnation moves on is something that can catch any DBA but is one of those fun little advanced topics, along with Data Guard, that adds a little spice to the day.

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