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V$ASM_DISKGROUP displays information from the header of ASM disks

Posted by Kamran Agayev A. on January 17th, 2014

While playing with OCR recovery, suddenly I realized that V$ASM_DISKGROUP view gets information from the headers of the ASM disk files that are specified at *.ASM_DISKSTRING parameter. Here’s the description of V$ASM_DISKGROUP view from documentation:

V$ASM_DISKGROUP displays one row for every ASM disk group discovered by the ASM instance on the node.

http://docs.oracle.com/cd/E18283_01/server.112/e17110/dynviews_1027.htm

 

I got explain plan of V$ASM_DISKGROUP to know which X$ table stand behind it and got – X$KFGRP

SQL> set autotrace on

SQL> select count(1) from v$asm_diskgroup;

COUNT(1)

———-

3

 

pic1

 

 

 

 

 

 

 

SQL> select name_kfgrp from x$kfgrp;

NAME_KFGRP

——————————

 

DATA

FLASH

OCR

 

SQL> select GRPNUM_KFDSK, NUMBER_KFDSK, STATE_KFDSK, ASMNAME_KFDSK, PATH_KFDSK from x$KFDSK;

pic2

 

 

 

 

 

 

 

 

Then I queried ASM_DISKGROUPS parameter :

 

SQL> show parameter disk

 

NAME                                 TYPE        VALUE

———————————— ———– ——————————

asm_diskgroups                       string      OCR, DATA, FLASH

asm_diskstring                       string      /dev/oracleasm/disks

 

No I will create a new tablespace under FLASH diskgroup, create a new table, change owner of the disk of FLASH diskgroup and make it #*disappear* from V$ASM_DISKGROUP view, and then return everything back 

SQL> create tablespace new_tbs datafile ‘+FLASH’;

Tablespace created.

SQL> create table new_table (id number) tablespace new_tbs;

Table created.

SQL> insert into new_table values(1);

1 row created.

SQL> commit;

Commit complete.

SQL> select * from new_table;

ID
———-
1

 

I create a parameter file from spfile, change ASM_DISKGROUPS parameter to OCR,DATA (remove FLASH) and mount the ASM instance again using a parameter file with

ASM_DISKGROUPS=’OCR’,'DATA’ specified:

 

pic7

 

So DISK6 is member and as the disk is discovered by ASM instance, FLASH diskgroup is dismounted, but still there.

Let’s change the owner of the disk and check it again. But before checking the owner, let’s read it’s header by KFED:

[root@node1 disks]# kfed read DISK6

pic4

 

Now let’s start the instance and check V$ASM_DISKGROUP view:
pic5

 

Query X$KFGRP view:


 
SQL> select NAME_KFGRP from X$KFGRP;

NAME_KFGRP

——————————

 

DATA

OCR

SQL>

 

Switch to the database and check if you can query the table:

SQL> select * from new_table;

select * from new_table

*

ERROR at line 1:

ORA-01157: cannot identify/lock data file 8 – see DBWR trace file

ORA-01110: data file 8: ‘+FLASH/rac/datafile/new_tbs.257.837080939′

SQL>

 

 

Now shutdown the ASM instance, return the owner back and check V$ASM_DISKGROUP again:

[root@node1 disks]# chown -R oracle:dba DISK6
pic6

 

FLASH diskgroup appeared, however it’s not specified at ASM_DISKGROUPS parameter. Now mount the diskgroup and query the table again:

SQL> alter diskgroup flash mount;

Diskgroup altered.

SQL>

SQL> select * from new_table;

ID
———-
1

SQL>

 

This means that if you want to move the ASM instance to another host, it’s enough to specify ASM_DISKSTRING parameter, V$ASM_DISKGROUP will discover all diskgroups

One Response to “V$ASM_DISKGROUP displays information from the header of ASM disks”

  1. vanita Says:

    nice article

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