Resolving Gaps in Data Guard Apply Using Incremental RMAN BAckup

Recently, we had a glitch on a Data Guard (physical standby database) on infrastructure. This is not a critical database; so the monitoring was relatively lax. And that being done by an outsourcer does not help it either. In any case, the laxness resulted in a failure remaining undetected for quite some time and it was eventually discovered only when the customer complained. This standby database is usually opened for read only access from time to time.This time, however, the customer saw that the data was significantly out of sync with primary and raised a red flag. Unfortunately, at this time it had become a rather political issue.

Since the DBA in charge couldn’t resolve the problem, I was called in. In this post, I will describe the issue and how it was resolved. In summary, there are two parts of the problem:

(1) What happened
(2) How to fix it

What Happened

Let’s look at the first question – what caused the standby to lag behind. First, I looked for the current SCN numbers of the primary and standby databases. On the primary:

SQL> select current_scn from v$database;

CURRENT_SCN
-----------
1447102

On the standby:

SQL> select current_scn from v$database;

CURRENT_SCN
-----------
1301571

Clearly there is a difference. But this by itself does not indicate a problem; since the standby is expected to lag behind the primary (this is an asynchronous non-real time apply setup). The real question is how much it is lagging in the terms of wall clock. To know that I used the scn_to_timestamp function to translate the SCN to a timestamp:

SQL> select scn_to_timestamp(1447102) from dual;

SCN_TO_TIMESTAMP(1447102)
-------------------------------
18-DEC-09 08.54.28.000000000 AM

I ran the same query to know the timestamp associated with the SCN of the standby database as well (note, I ran it on the primary database, though; since it will fail in the standby in a mounted mode):

SQL> select scn_to_timestamp(1301571) from dual;

SCN_TO_TIMESTAMP(1301571)
-------------------------------
15-DEC-09 07.19.27.000000000 PM

This shows that the standby is two and half days lagging! The data at this point is not just stale; it must be rotten.

The next question is why it would be lagging so far back in the past. This is a 10.2 database where FAL server should automatically resolved any gaps in archived logs. Something must have happened that caused the FAL (fetch archived log) process to fail. To get that answer, first, I checked the alert log of the standby instance. I found these lines that showed the issue clearly:

…
Fri Dec 18 06:12:26 2009
Waiting for all non-current ORLs to be archived...
Media Recovery Waiting for thread 1 sequence 700
Fetching gap sequence in thread 1, gap sequence 700-700
… …
Fri Dec 18 06:13:27 2009
FAL[client]: Failed to request gap sequence
GAP - thread 1 sequence 700-700
DBID 846390698 branch 697108460
FAL[client]: All defined FAL servers have been attempted.
…

Going back in the alert log, I found these lines:

Tue Dec 15 17:16:15 2009
Fetching gap sequence in thread 1, gap sequence 700-700
Error 12514 received logging on to the standby
FAL[client, MRP0]: Error 12514 connecting to DEL1 for fetching gap sequence
Tue Dec 15 17:16:15 2009
Errors in file /opt/oracle/admin/DEL2/bdump/del2_mrp0_18308.trc:
ORA-12514: TNS:listener does not currently know of service requested in connect descriptor
Tue Dec 15 17:16:45 2009
Error 12514 received logging on to the standby
FAL[client, MRP0]: Error 12514 connecting to DEL1 for fetching gap sequence
Tue Dec 15 17:16:45 2009
Errors in file /opt/oracle/admin/DEL2/bdump/del2_mrp0_18308.trc:
ORA-12514: TNS:listener does not currently know of service requested in connect descriptor

This clearly showed the issue. On December 15th at 17:16:15, the Managed Recovery Process encountered an error while receiving the log information from the primary. The error was ORA-12514 “TNS:listener does not currently know of service requested in connect descriptor”. This is usually the case when the TNS connect string is incorrectly specified. The primary is called DEL1 and there is a connect string called DEL1 in the standby server.

The connect string works well. Actually, right now there is no issue with the standby getting the archived logs; so there connect string is fine - now. The standby is receiving log information from the primary. There must have been some temporary hiccups causing that specific archived log not to travel to the standby. If that log was somehow skipped (could be an intermittent problem), then it should have been picked by the FAL process later on; but that never happened. Since the sequence# 700 was not applied, none of the logs received later – 701, 702 and so on – were applied either. This has caused the standby to lag behind since that time.

So, the fundamental question was why FAL did not fetch the archived log sequence# 700 from the primary. To get to that, I looked into the alert log of the primary instance. The following lines were of interest:

…
Tue Dec 15 19:19:58 2009
Thread 1 advanced to log sequence 701 (LGWR switch)
Current log# 2 seq# 701 mem# 0: /u01/oradata/DEL1/onlinelog/o1_mf_2_5bhbkg92_.log
Tue Dec 15 19:20:29 2009Errors in file /opt/oracle/product/10gR2/db1/admin/DEL1/bdump/del1_arc1_14469.trc:
ORA-00308: cannot open archived log '/u01/oraback/1_700_697108460.dbf'
ORA-27037: unable to obtain file status
Linux Error: 2: No such file or directory
Additional information: 3
Tue Dec 15 19:20:29 2009
FAL[server, ARC1]: FAL archive failed, see trace file.
Tue Dec 15 19:20:29 2009
Errors in file /opt/oracle/product/10gR2/db1/admin/DEL1/bdump/del1_arc1_14469.trc:
ORA-16055: FAL request rejected
ARCH: FAL archive failed.
Archiver continuing
Tue Dec 15 19:20:29 2009
ORACLE Instance DEL1 - Archival Error. Archiver continuing.
…

These lines showed everything clearly. The issue was:

ORA-00308: cannot open archived log '/u01/oraback/1_700_697108460.dbf'
ORA-27037: unable to obtain file status
Linux Error: 2: No such file or directory

The archived log simply was not available. The process could not see the file and couldn’t get it across to the standby site.

Upon further investigation I found that the DBA actually removed the archived logs to make some room in the filesystem without realizing that his action has removed the most current one which was yet to be transmitted to the remote site. The mystery surrounding why the FAL did not get that log was finally cleared.

Solution

Now that I know the cause, the focus was now on the resolution. If the archived log sequence# 700 was available on the primary, I could have easily copied it over to the standby, registered the log file and let the managed recovery process pick it up. But unfortunately, the file was gone and I couldn’t just recreate the file. Until that logfile was applied, the recovery will not move forward. So, what are my options?

One option is of course to recreate the standby - possible one but not technically feasible considering the time required. The other option is to apply the incremental backup of primary from that SCN number. That’s the key – the backup must be from a specific SCN number. I have described the process since it is not very obvious. The following shows the step by step approach for resolving this problem. I have shown where the actions must be performed – [Standby] or [Primary].

1. [Standby] Stop the managed standby apply process:

SQL> alter database recover managed standby database cancel;

Database altered.

2. [Standby] Shutdown the standby database

3. [Primary] On the primary, take an incremental backup from the SCN number where the standby has been stuck:

RMAN> run {
2> allocate channel c1 type disk format '/u01/oraback/%U.rmb';
3> backup incremental from scn 1301571 database;
4> }

using target database control file instead of recovery catalog
allocated channel: c1
channel c1: sid=139 devtype=DISK

Starting backup at 18-DEC-09
channel c1: starting full datafile backupset
channel c1: specifying datafile(s) in backupset
input datafile fno=00001 name=/u01/oradata/DEL1/datafile/o1_mf_system_5bhbh59c_.dbf
… …
piece handle=/u01/oraback/06l16u1q_1_1.rmb tag=TAG20091218T083619 comment=NONE
channel c1: backup set complete, elapsed time: 00:00:06
Finished backup at 18-DEC-09
released channel: c1

4. [Primary] On the primary, create a new standby controlfile:

SQL> alter database create standby controlfile as '/u01/oraback/DEL1_standby.ctl';

Database altered.

5. [Primary] Copy these files to standby host:

oracle@oradba1 /u01/oraback# scp *.rmb *.ctl oracle@oradba2:/u01/oraback
oracle@oradba2's password:
06l16u1q_1_1.rmb 100% 43MB 10.7MB/s 00:04
DEL1_standby.ctl 100% 43MB 10.7MB/s 00:04

6. [Standby] Bring up the instance in nomount mode:

SQL> startup nomount

7. [Standby] Check the location of the controlfile:

SQL> show parameter control_files

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
control_files string /u01/oradata/standby_cntfile.ctl

8. [Standby] Replace the controlfile with the one you just created in primary.

9. $ cp /u01/oraback/DEL1_standby.ctl /u01/oradata/standby_cntfile.ctl

10.[Standby] Mount the standby database:

SQL> alter database mount standby database;

11.[Standby] RMAN does not know about these files yet; so you must let it know – by a process called cataloging. Catalog these files:

$ rman target=/

Recovery Manager: Release 10.2.0.4.0 - Production on Fri Dec 18 06:44:25 2009

Copyright (c) 1982, 2007, Oracle. All rights reserved.

connected to target database: DEL1 (DBID=846390698, not open)
RMAN> catalog start with '/u01/oraback';

using target database control file instead of recovery catalog
searching for all files that match the pattern /u01/oraback

List of Files Unknown to the Database
=====================================
File Name: /u01/oraback/DEL1_standby.ctl
File Name: /u01/oraback/06l16u1q_1_1.rmb

Do you really want to catalog the above files (enter YES or NO)? yes
cataloging files...
cataloging done

List of Cataloged Files
=======================
File Name: /u01/oraback/DEL1_standby.ctl
File Name: /u01/oraback/06l16u1q_1_1.rmb

12.Recover these files:

RMAN> recover database;

Starting recover at 18-DEC-09
using channel ORA_DISK_1
channel ORA_DISK_1: starting incremental datafile backupset restore
channel ORA_DISK_1: specifying datafile(s) to restore from backup set
destination for restore of datafile 00001: /u01/oradata/DEL2/datafile/o1_mf_system_5lptww3f_.dbf
...…
channel ORA_DISK_1: reading from backup piece /u01/oraback/05l16u03_1_1.rmb
channel ORA_DISK_1: restored backup piece 1
piece handle=/u01/oraback/05l16u03_1_1.rmb tag=TAG20091218T083619
channel ORA_DISK_1: restore complete, elapsed time: 00:00:07

starting media recovery

archive log thread 1 sequence 8012 is already on disk as file /u01/oradata/1_8012_697108460.dbf
archive log thread 1 sequence 8013 is already on disk as file /u01/oradata/1_8013_697108460.dbf
… …

13. After some time, the recovery fails with the message:

archive log filename=/u01/oradata/1_8008_697108460.dbf thread=1 sequence=8009
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03002: failure of recover command at 12/18/2009 06:53:02
RMAN-11003: failure during parse/execution of SQL statement: alter database recover logfile '/u01/oradata/1_8008_697108460.dbf'
ORA-00310: archived log contains sequence 8008; sequence 8009 required
ORA-00334: archived log: '/u01/oradata/1_8008_697108460.dbf'

This happens because we have come to the last of the archived logs. The expected archived log with sequence# 8008 has not been generated yet.

14.At this point exit RMAN and start managed recovery process:

SQL> alter database recover managed standby database disconnect from session;

Database altered.

15.Check the SCN’s in primary and standby:

[Standby] SQL> select current_scn from v$database;

CURRENT_SCN
-----------
1447474
[Primary] SQL> select current_scn from v$database;

CURRENT_SCN
-----------
1447478
Now they are very close to each other. The standby has now caught up.