ASM

History

ASM was first released with Oracle 10g Release 1.

[edit]Features

• ASM takes control of disk devices (no disk partitioning needed)
• More space can be transparently added
• I/Os are spread over all the available disks (striping)
• ASM can maintain redundant copies of the data (mirror)
• Storage can be migrated between disks
• Same disk (or set of disks) can be shared amongst many DBs

[edit]Installing ASM

It is recommended to install ASM in a separate Oracle home so that upgrades can be done independently from the databases.

Use the DBCA utility to create the ASM instance.

[edit]Starting and stopping

Start the ASM instance:

$ export ORACLE_SID=+ASM
$ sqlplus "/ as sysdba"

SQL*Plus: Release 10.2.0.1.0 - Production on Sun Sep 3 00:28:09 2006
Copyright (c) 1982, 2005, Oracle.  All rights reserved.
Connected to an idle instance.
SQL> startup
ASM instance started
Total System Global Area   83886080 bytes
Fixed Size                  1217836 bytes
Variable Size              57502420 bytes
ASM Cache                  25165824 bytes
ASM diskgroups mounted

Stop the ASM instance:

$ export ORACLE_SID=+ASM
$ sqlplus "/ as sysdba"
shutdown immediate

[edit]Adding diskgroups

SQL> create diskgroup orag2 external redundancy disk 'ORCL:VOL5';
Diskgroup created.

SQL> select group_number,disk_number,mode_status,name from v$asm_disk;
GROUP_NUMBER DISK_NUMBER MODE_STATUS    NAME
------------ ----------- -------------- -------------------------------------
           0           5 ONLINE
           1           0 ONLINE         VOL1
           1           1 ONLINE         VOL2
           1           2 ONLINE         VOL3
           1           3 ONLINE         VOL4
           2           0 ONLINE         VOL5
6 rows selected.

[edit]Recreating a diskgroup

All ASM metadata must be cleared before attempting to re-create a diskgroup. Example Command:

dd if=/dev/zero of=/dev/rdsk/c1t4d0s4 bs=8192 count=12800

When done, restart ASM and create the diskgroup from scratch.

[edit]Rebalancing

The rebalancing speed is controlled by the ASM_POWER_LIMIT initialization parameter. Setting it to 0 will disable disk rebalancing.

To force rebalancing of a diskgroup:

ALTER DISKGROUP data REBALANCE POWER 11 WAIT;

[edit]Converting to ASM

One can use Rman to convert a datafile, tablespace or entire database from/to ASM. Here are the steps required to migrate an entire database to ASM:

[edit]Convert a database to ASM

Ensure the database is using an SPFILE and not a PFILE (it's about time after all!). Set parameters on the target database. For example, if we set both DB_CREATE_FILE_DEST and DB_RECOVERY_FILE_DEST we should get mirrored controlfiles and duplexed log files by default:

SQL> alter system set DB_CREATE_FILE_DEST = '+DATA';
SQL> alter system set DB_RECOVERY_FILE_DEST_SIZE = 17G;
SQL> alter system set DB_RECOVERY_FILE_DEST = '+RECOVER';
SQL> alter system set CONTROL_FILES = '+DATA';

Start the database in NOMOUNT mode and restore the controlfile into the new location from the old location:

RMAN> connect target /
RMAN> STARTUP NOMOUNT
RMAN> RESTORE CONTROLFILE FROM 'old_control_file_name';

Mount the database and copy the database into the ASM disk group:

RMAN> ALTER DATABASE MOUNT;
RMAN> CONFIGURE DEVICE TYPE DISK PARALLELISM 8;
RMAN> BACKUP AS COPY DATABASE FORMAT '+DATA';

Switch all datafiles to the new ASM location and open the database:

RMAN> SWITCH DATABASE TO COPY;
RMAN> ALTER DATABASE OPEN;

Add new tempfiles and drop the old tempfiles:

SQL> alter tablespace temp add tempfile;
SQL> alter database tempfile '...' DROP;
SQL> select * from dba_temp_files; 

Optionally, move SPFILE into ASM:

SQL> CREATE SPFILE '+DATA' FROM PFILE;

Move redo log files into ASM - for each group:

SQL> ALTER DATABASE DROP LOGFILE GROUP 1;
SQL> ALTER DATABASE ADD LOGFILE GROUP 1 SIZE 100M;

If a logfile is active and cannot be dropped, issue an ALTER DATABASE SWITCH LOGFILE; command and try again.

[edit]Convert a tablespace to ASM

Ensure the database in in archive log mode, and from rman:

connect target;
sql "alter tablespace TSNAME offline";
backup as copy tablespace TSNAME format '+DATA';
switch tablespace TSNAME to copy;
sql "alter tablespace TSNAME online";
exit;

[edit]Convert a datafile to ASM

Ensure the database in in archive log mode, and from rman:

connect target;
sql "alter database datafile '...' offline";
backup as copy datafile '...' format '+DATA';
switch datafile '..' to copy;
sql "alter database datafile '...' online";
exit;

[edit]Monitoring

Some of the views that can be used to monitor ASM configuration and activity:

• V$ASM_DISK - ASM disks
• V$ASM_DISK_STAT - cached view of V$ASM_DISK for faster access (used by Enterprise Manager)
• V$ASM_DISKGROUP - ASM diskgroups
• V$ASM_DISKGROUP_STAT - cached view of V$ASM_DISKGROUP for faster access (used by Enterprise Manager)
• V$ASM_OPERATION - status of ongoing disk operations (like rebalancing)