Apps DBA Diaries: Oracle ASM Interview Questions for the experienced

Friday, 22 November 2024

Click Here for ASM Interview Questions - Part 1

Expected Answer: ASM rebalance is the process of redistributing data across the disks in a disk group when there is a change in the disk group (e.g., adding or dropping a disk). The rebalance operation ensures that data is evenly spread across the available disks to optimize performance and storage. It occurs automatically when disk group changes are made and can be monitored with the v$asm_operation view.

Expected Answer: When a disk fails in a redundant disk group (using mirroring), Oracle ASM automatically rebalances the data to the remaining healthy disks. If you are using external redundancy, you may need to rely on external RAID for recovery. ASM detects disk failures via periodic disk checks and logs the failure, making it easy for administrators to take action, such as adding a replacement disk.

Expected Answer: To migrate data from one ASM disk group to another, you can:
1. Use the ALTER DISKGROUP command to move data:
```
ALTER DISKGROUP <source_diskgroup> MOVE <file_name> TO <target_diskgroup>;
```
2. Use DBMS_FILE_TRANSFER or other tools like RMAN for moving data files between disk groups.
3. Alternatively, you can use Data Pump for migrating large datasets.

Expected Answer: To recover from a disk failure in ASM:
1. Identify the failed disk using V$ASM_DISK.
2. Ensure that the disk group is still operational (in case of mirroring, data is still available on the other disks).
3. Replace the failed disk physically.
4. Add the new disk to the ASM disk group using ALTER DISKGROUP ADD DISK.
5. Oracle ASM will automatically rebalance the data across the disks, ensuring data is mirrored correctly.

Expected Answer: The ASM instance manages the physical storage (disk groups and disks) and provides the storage abstraction for the Oracle database. It operates independently from the Oracle database instance, which connects to the ASM instance for reading/writing data files, control files, and redo logs. The database instance communicates with the ASM instance via Oracle background processes (e.g., DBWR, LGWR).

Expected Answer: ASM is a software-based storage management solution that operates within the Oracle Database ecosystem. While it provides features similar to RAID (redundancy, striping, etc.), it is tightly integrated with Oracle databases and handles file management and storage distribution automatically. RAID, on the other hand, is a hardware or software-based technology used for disk redundancy and performance at the hardware level, but it lacks the database-level integration that ASM offers.

Expected Answer: Yes, Oracle RAC can be configured with ASM for shared storage across multiple nodes. In RAC, multiple database instances run on different nodes, and ASM provides shared disk storage, which ensures that all instances have access to the same database files stored in ASM disk groups. ASM simplifies the storage configuration for RAC by handling disk management in a cluster environment.

Expected Answer: Some key ASM parameters for tuning performance include:
- ASM_DISK_REPAIR_TIME: Defines the time allowed for disk repairs.
- ASM_POWER_LIMIT: Controls the amount of CPU resources ASM can use during rebalancing.
- ASM_DISKGROUP_REPAIR_TIME: Specifies the time allowed for repairing the disk group in case of a failure.

Expected Answer: You can monitor ASM performance using the following methods:
- V$ASM views: Use views like V$ASM_DISK, V$ASM_DISKGROUP, and V$ASM_OPERATION to track ASM performance and disk operations.
- Oracle Enterprise Manager (OEM): OEM provides a graphical interface to monitor ASM performance, including disk group usage, rebalance status, and storage health.

Apps DBA Diaries