Skip to main content

Don't trust the lying (Data Guard) broker

One of the new 12c features is the "VALIDATE DATABASE" command. According to the documentation it should do many thorough checks and tell you if all is configured well and correctly. However, there is one catch - or to put it a little more bluntly - bug. Or two.

You know that you need standby redo logs for SYNC (or the new FASTSYNC) transport mode. The validate command knows that, too. And you know that you should have one more standby redo log than online redo logs. The validate command seems to know this one as well.

However, the checks appear to have one flaw: they test whether the threads (and let's talk here about a single-instance, so we have only thread #1) have enough standby redo logs (SRLs) assigned. But when you create an SRL with 'alter database add standby logfile', they are unassigned to any thread. In fact, you get 0 as thread#:

select thread#, sequence# from V$STANDBY_LOG;

THREAD# SEQUENCE#
------- ---------
      0         0
      0         0
      0         0
      0         0
Which is perfectly fine - Oracle waits until the instance actually needs the SRL and only then is this assigned. Makes the administration easier.

But the guys responsible for VALIDATE DATABASE do not seem to realize this. So if you have just set up your SRLs and run the validate command - just to see if the config is all ok (e.g. because you just want to change the LogXptMode and protection mode) then you will get a result like this:
Thread #  Online Redo Log Groups  Standby Redo Log Groups Status
              (CDB5)                  (CDB5SBY)
    1         3                       0                       Insufficient SRLs
    Warning: standby redo logs not configured for thread 1 on CDB5SBY

WTF? Yes, the validate command did not understand that we have plenty of SRLs, only that they have not yet been assigned to any thread.

So.. we do a switchover, back and forth, to let both databases touch the SRLs and...

Thread #  Online Redo Log Groups  Standby Redo Log Groups Status
              (CDB5)                  (CDB5SBY)
    1         3                       2                       Insufficient SRLs

And we still receive a warning - although we have created 4 SRLs, only two of which Oracle has required so far...with the other two currently unassigned. Again, VALIDATE DATABASE is not aware of this and complains.

The morale? Don't just trust the command, especially in the beginning, when your configuration is fresh and still settling down. Although that's exactly the time you want to use checks like this.

Comments

Popular posts from this blog

ORA-27048: skgfifi: file header information is invalid

I was asked to analyze a situation, when an attempt to recover a 11g (standby) database resulted in bunch of "ORA-27048: skgfifi: file header information is invalid" errors.

I tried to reproduce the error on my test system, using different versions (EE, SE, 11.1.0.6, 11.1.0.7), but to no avail. Fortunately, I finally got to the failing system:

SQL> recover standby database;
ORA-00279: change 9614132 generated at 11/27/2009 17:59:06 needed for thread 1
ORA-00289: suggestion :
/u01/flash_recovery_area/T1/archivelog/2009_11_27/o1_mf_1_208_%u_.arc
ORA-27048: skgfifi: file header information is invalid
ORA-27048: skgfifi: file header information is invalid
ORA-27048: skgfifi: file header information is invalid
ORA-27048: skgfifi: file header information is invalid
ORA-27048: skgfifi: file header information is invalid
ORA-27048: skgfifi: file header information is invalid
ORA-00280: change 9614132 for thread 1 is in sequence #208


Interestingly, nothing interesting is written to alert.log n…

Reading data from PGA and SGA

Overview For our investigation of execution plan as it is stored in memory, we need in the first place to be able to read the memory.

We have the options of
x$ksmmem, reading SGA using SQL. Personally I don't like it, it's cumbersome and slow.direct SGA read: obviously reading SGA only; it's fast and easy to doread process memory: can read PGA, process stack - and since the processes do map the SGA, too, you can read it as well. Unfortunately ptrace sends signals to the processes and the process is paused when reading it, but so far all my reads were short and fast and the processes did not notice. Some OS configurations can prevent you from using ptrace (e.g. docker by default), google for CAP_SYS_PTRACE.gdb: using your favorite debugger, you can read memory as well. Useful when investigating. Direct SGA read I always considered direct SGA read of some dark magic, but the fundamentals are actually very easy. It still looks like sorcery when actually reading the Oracle in…

Reading execution plan from SGA and PGA - teaser

Some of you have seen my presentation about hidden parts of Oracle execution plans and how to access the plan in the memory directly and parse it. I presented it at OakTable World 2017  and it will be also presented in Wellington and Acukland this November.  You can download the presentation at http://vitspinka.com/files/ReadingPlanFromSGA-OTWatOOW-2017.pdf.


I realize that many people did not have the chance to attend... and that the slides need quite a lot of explaining, it's hard to understand this internals without more explanation.


Thus you can look forward to seeing a handful of blog posts, inspired by this presentation, and explain some of the aspects of this whole topic.

We will start with some basic tools; accessing SGA and PGA, which you may find useful for many other tasks, too. Then we will look at some details of the execution plan. This is not - and neither the presentation is - an exhaustive guide to the execution plan internals. It would be a multi-year project to t…