Automatic and Manual ASM Disk Repair in 11g

Oracle 11g ASM is capable of repairing broken blocks on the fly when reading them. When Oracle finds a broken block, it attempts to read it from the mirror copy of the respective extent and repairs the broken version automatically. This can only happen on disk groups with at least normal redundancy and it only happens for those blocks which are read.

The new remap command of asmcmd can be used to recover a range of unreadable blocks. ASM would read a bad block from the good copy and restore it to a new location on the disk.  This can also be run from the enterprise manager interface.

ASM Disk Group Checks Enhanced

The check command was already available in Oracle ASM 10g. It was necessary there to specify what to check with the check command. As of Oracle ASM 11g, this command has been simplified. It is not necessary to specify any more what to check because the command makes all available checks in one go now. The CHECK keyword performs the following operations without the need for specifications:

• Checks if the disk is consistent

• Cross checks the file extent maps and allocation tables for consistency

• Checks if the metadata of the directories for alias and files are linked correctly

• Checks if the directory tree of the aliases is linked correctly.

• Checks that ASM metadata directories do not have unreadable allocated blocks.

SYS AS SYSDBA @ +ASM SQL> ALTER DISKGROUP data CHECK;

The above as well as the below command issued in an ASM instance would check all disks in the disk group and add a message into the ASM instance's alert log file with information about the successful run of the check and about found broken blocks. In addition, these commands would also recover the bad blocks. The repair option is the DEFAULT for the check command.

SYS AS SYSDBA @ +ASM SQL> ALTER DISKGROUP data CHECK REPAIR; 

The next command would only check for bad blocks but not recover them.

SYS AS SYSDBA @ +ASM SQL> ALTER DISKGROUP data CHECK NOREPAIR;

[oracle@rhas4 ~]$ tail -f /u01/app/oracle/diag/asm/+asm/+ASM/alert/log.xml

The below listing shows the message in the xml version of the alter log of the ASM instance:

 <txt>SUCCESS: check of diskgroup DATA found no errors
 </txt>
</msg>
<msg time='2007-10-22T10:26:33.700+02:00' org_id='oracle' comp_id='asm'
 client_id='' type='UNKNOWN' level='16'
 host_id='rhas4.mydomain' host_addr='192.168.1.1' module='OMS'
 pid='12810'>
 <txt>SUCCESS: ALTER DISKGROUP DATA CHECK
 </txt>
</msg>

The full check for all metadata packages into one single command makes the checks easier, but the drawback is the additional overhead which is burdening the I/O subsystem now and might slow down performance.