How to Safely Remove a Used Disk from an ASM Disk Group in Oracle 19c RAC
Removing or deleting a used disk from an ASM disk group in Oracle 19c RAC without impacting data requires proper planning and validation. ASM provides built-in rebalancing capabilities, but certain checks must be completed to ensure data integrity and system stability.
This article walks through a step-by-step procedure to safely remove a disk from an ASM disk group.
Key Points to Remember
In Normal or High Redundancy disk groups, ASM automatically redistributes data across remaining disks.
For External Redundancy, ensure sufficient free space is available before removing a disk.
FREE_MB must be greater than the space used by the disk you plan to drop.
REQUIRED_MIRROR_FREE_MB must be less than FREE_MB to maintain proper mirroring.
ASM Rebalance Power controls the speed of data redistribution when disks are added or removed.
Rebalance Power Example
ALTER DISKGROUP DATA REBALANCE POWER 1;
Recommended range: 1–11
Maximum possible value: 0–1024, depending on system resources
Higher values increase resource usage (CPU & I/O)
Example Scenario
We have an ASM disk group named +DATA with multiple disks.
The goal is to safely remove one disk: DS8.
Step 1: Check Existing Disks in the Disk Group
SET LINES 200
COL name FORMAT a20
COL path FORMAT a28
COL total_mb FORMAT 99999
COL free_mb FORMAT 99999
SELECT group_number,
name,
path,
total_mb,
free_mb,
header_status,
mount_status,
state
FROM v$asm_disk
WHERE group_number = (
SELECT group_number
FROM v$asm_diskgroup
WHERE name = ‘DATA’
);
Sample Output
GROUP_NUMBER NAME PATH TOTAL_MB FREE_MB HEADER_STATUS MOUNT_STATUS STATE
———— ———– ———– ——– ——- ————- ———— ——
2 DS8 /tst/asm 2048 848 MEMBER CACHED NORMAL
2 DATA_0000 /tst/asm 20480 20480 MEMBER CACHED NORMAL
Step 2: Check Disk Group Redundancy and Space
SELECT name,
type,
state,
total_mb,
free_mb,
required_mirror_free_mb,
usable_file_mb
FROM v$asm_diskgroup
WHERE name = ‘DATA’;
Sample Output
NAME TYPE STATE TOTAL_MB FREE_MB REQUIRED_MIRROR_FREE_MB USABLE_FILE_MB
—– —— ——– ——– ——- ———————– ————–
DATA EXTERN MOUNTED 20480 19000 0 16000
Ensure the disk group has enough free space before proceeding.
SELECT name, free_mb, required_mirror_free_mb
FROM v$asm_diskgroup
WHERE name = ‘DATA’;
Step 3: Drop the Disk from ASM Disk Group
Run the following command to remove the disk and allow ASM to rebalance the data automatically:
ALTER DISKGROUP DATA DROP DISK DS8;
Alternative Commands (Use with Caution)
ALTER DISKGROUP DATA_DG DROP DISK DS1;
DROP DISKGROUP DATA_DG INCLUDING CONTENTS;
The second command removes the entire disk group and should only be used when decommissioning it completely.
Step 4: Monitor the Rebalancing Process
ASM redistributes data across remaining disks during rebalancing.
SELECT group_number,
operation,
state,
est_minutes
FROM v$asm_operation;
Monitor until STATE shows COMPLETED
If no rows are returned, rebalancing has already finished
Step 5: Verify Disk Group Status After Rebalancing
SELECT name, total_mb, free_mb
FROM v$asm_diskgroup
WHERE name = ‘DATA’;
Sample Output
NAME TOTAL_MB FREE_MB
—– ——– ——-
DATA 20480 17272
Step 6: Confirm Disk Removal from ASM
SELECT name, path
FROM v$asm_disk
WHERE group_number = (
SELECT group_number
FROM v$asm_diskgroup
WHERE name = ‘DATA’
);
Sample Output
NAME PATH
———- —————-
DATA_0000 /tst/asmK
If the disk is no longer listed, it has been successfully removed.
Step 7: Decommission the Physical Disk (Optional)
Check disks that are no longer part of any ASM disk group:
SELECT disk_number, name, path
FROM v$asm_disk
WHERE header_status = ‘FORMER’;
Sample Output
DISK_NUMBER NAME PATH
———– —– —————-
1 /tst/asm/DS05
0 /tst/asm/DS08
Disks with HEADER_STATUS = ‘FORMER’ are available for reuse.
Step 8: Clear ASM Metadata (If Reusing the Disk)
To reuse the disk for another ASM disk group or a different purpose:
dd if=/dev/zero of=/dev/sdX bs=1024 count=100
or for AFD disks:
asmcmd afd_label <disk> –init
Step 9: Remove Disk from ASM Configuration (ASMLib)
oracleasm deletedisk DS8
Step 10: Final Verification
Verify disk group health and ensure no impact on the database:
SELECT *
FROM v$asm_diskgroup
WHERE name = ‘DATA’;
Confirm:
Disk group state is MOUNTED
No offline disks
No errors in ASM or database alert logs
Conclusion
By following these steps, you can safely remove a used disk from an ASM disk group in Oracle 19c RAC without data loss or service disruption. ASM’s rebalancing mechanism ensures data remains available and properly distributed across remaining disks.
