Nov 17

Where Are My Nodes?

I should mention the current setup is a 4 node RAC database with a dataguard managed standby comprised of a 3 node RAC database cluster. The current database version is

While writing a shell script today, I needed to know from connecting to a database in a cluster what all the related nodes were. This isn’t a big deal, I could simply query gv$instance to pull a list of the host names, and the associated instances.

SELECT host_name, instance_name, instance_number
    FROM gv$instance
ORDER BY instance_number

--------------- --------------- ---------------
homeOELdb01     orapdb1                       1
homeOELdb02     orapdb2                       2
homeOELdb03     orapdb3                       3
homeOELdb04     orapdb4                       4

But what about the standby nodes? I can query v$archive_dest_status

SELECT destination
 WHERE status = 'VALID' AND type = 'PHYSICAL'


Ok, great, that is the name of my standby database cluster, but there are three nodes! I want to see what is going where! I could use dgmgrl,

DGMGRL> show configuration

Configuration - orapdb_dgconf

  Protection Mode: MaxPerformance
    orapdb - Primary database
    orasdb - Physical standby database

Fast-Start Failover: DISABLED

Configuration Status:

DGMGRL> show database 'orasdb'

Database - orasdb

  Role:            PHYSICAL STANDBY
  Intended State:  APPLY-ON
  Transport Lag:   0 seconds (computed 0 seconds ago)
  Apply Lag:       0 seconds (computed 0 seconds ago)
  Apply Rate:      21.80 MByte/s
  Real Time Query: OFF
    orasdb1 (apply instance)

Database Status:

So that gives us the databases in the standby, but parsing this out would be a pain. I just want to generate a report, and it is much easier to be able to query the database to generate my output. However I couldn’t find a place to do this!

Enter the X$DRC view. I could find very little about this view itself. With any X$ table, the view is rather cryptic, but I was able to get enough information out of it to solve my problem.

 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 ADDR                                               RAW(8)
 INDX                                               NUMBER
 INST_ID                                            NUMBER
 OBJECT_ID                                          NUMBER
 ATTRIBUTE                                          VARCHAR2(30)
 VALUE                                              VARCHAR2(512)
 PARENT_ID                                          VARCHAR2(15)
 STATUS                                             VARCHAR2(30)
 MESSAGE                                            VARCHAR2(256)
 ERRNUM                                             NUMBER
 VALUE_RAW                                          RAW(512)

Because there isn’t a lot of information on this table I had to make some assumptions, based on the data

  SELECT inst_id,
ORDER BY inst_id, object_id, parent_id

   INST_ID  OBJECT_ID PARENT_ID       ATTRIBUTE                      VALUE
---------- ---------- --------------- ------------------------------ --------------------
         1       4096                 DRC                            orapdb_dgconf
         1       4096                 intended_state                 ONLINE
         1       4096                 protection_mode                MaxPerformance
         1       4096                 enabled                        YES
         1       4096                 fast_start_failover            DISABLED
         1   50331648 4096            SITE                           orapdb
         1   50397184 50331648        DATABASE                       orapdb
         1   50397184                 intended_state                 READ-WRITE-XPTON
         1   50397184                 role                           PRIMARY
         1   50397184                 enabled                        YES
         1   50397184                 RAC                            YES
         1   50397184                 connect_string                 orapdb
         1   50397185 50331648        INSTANCE                       orapdb4
         1   50397185                 host                           homeOELdb04
         1   50397186 50331648        INSTANCE                       orapdb2
         1   50397186                 host                           homeOELdb02
         1   50397187 50331648        INSTANCE                       orapdb1
         1   50397187                 host                           homeOELdb01
         1   50397188 50331648        INSTANCE                       orapdb3
         1   50397188                 host                           homeOELdb03
         1   67108864 4096            SITE                           orasdb
         1   67174400 67108864        DATABASE                       orasdb
         1   67174400                 connect_string                 orasdb
         1   67174400                 intended_state                 PHYSICAL-APPLY-ON
         1   67174400                 RAC                            YES
         1   67174400                 role                           PHYSICAL
         1   67174400                 enabled                        YES
         1   67174401 67108864        INSTANCE                       orasdb1
         1   67174401                 host                           homeOELdb01s
         1   67174402 67108864        INSTANCE                       orasdb2
         1   67174402                 host                           homeOELdb02s
         1   67174403 67108864        INSTANCE                       orasdb3
         1   67174403                 host                           homeOELdb03s

You can see the primary instances and their hosts listed above. To pull the standby databases out, I used the following query. I am guessing this could be made a lot cleaner, but I ran out of brain power while trying to optimize it, so here is the current query.

 WHERE     a.parent_id IN (SELECT object_id
                             FROM X$DRC
                            WHERE     VALUE NOT IN (SELECT db_unique_name
                                                      FROM v$database)
                                  AND attribute = 'SITE')
       AND a.attribute = 'INSTANCE'
       AND a.object_id = b.object_id
       AND b.ATTRIBUTE = 'host'

HOST                       INSTANCE
-------------------------- ----------
homeOELdb01s               orasdb1
homeOELdb02s               orasdb2
homeOELdb03s               orasdb3

I now have the information I want to include in my report. You could easily modify this to get all the nodes in the primary and the standby, but I would prefer to use gv$instance to get the primary nodes.

1/14/2016 Update

I didn’t notice this until much much later, if data guard is not enabled, The query on X$DRC will fail.

select * from X$DRC
ERROR at line 1:
ORA-16525: The Oracle Data Guard broker is not yet available.

I was using this to get my primary and standby nodes, and union them together, this was returning an empty set to me. I was spooling this information out to a text file to be used with a shell script, so my solution was to just break into two pieces, first spool out any RAC nodes, then check to see if there are any standby destinations enabled, and if so then spool out standby databases. This seems to work fine.

This is why it isn’t safe to use X$ tables, they aren’t documented and you may not get expected behavior out of them. I would have assumed I would just back zero rows, but that would be to assume the X$ table is like a V$ table. Not true, the X$ tables are used for reading portions of the oracle memory, mostly, but can also bull information like control file information into the memory to be read. So if the memory segment the data guard, or possibly the data guard broker filed don’t exist, this table cannot function.

To learn more about X$ tables, I suggest these three sites, they are my go to for X$ information

Tanner Poder: Oracle X$ tables – Part 1 – Where do they get their data from?

Morgan’s Library: Oracle X$ Structures

ORAFAQ: X$ Table naming structures


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