ASM Interview Questions & Answers

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1) What is ASM?

In Oracle Database 10g/11g there are two types of instances: database and ASM instances. The ASM instance, which is generally named +ASM, is started with the INSTANCE_TYPE=ASM init.ora parameter. This parameter, when set, signals the Oracle initialization routine to start an ASM instance and not a standard database instance. Unlike the standard database instance, the ASM instance contains no physical files; such as logfiles, controlfiles or datafiles, and only requires a few init.ora parameters for startup.

Upon startup, an ASM instance will spawn all the basic background processes, plus some new ones that are specific to the operation of ASM. The STARTUP clauses for ASM instances are similar to those for database instances. For example, RESTRICT prevents database instances from connecting to this ASM instance. NOMOUNT starts up an ASM instance without mounting any disk group. MOUNT option simply mounts all defined diskgroups

For RAC configurations, the ASM SID is +ASMx instance, where x represents the instance number.

2) What are the key benefits of ASM?

ASM provides filesystem and volume manager capabilities built into the Oracle database kernel. Withthis capability, ASM simplifies storage management tasks, such as creating/laying out databases and disk space management. Since ASM allows disk management to be done using familiar create/alter/drop SQL statements, DBAs do not need to learn a new skill set or make crucial decisions on provisioning.

The following are some key benefits of ASM:

• ASM spreads I/O evenly across all available disk drives to prevent hot spots and maximize performance.
• ASM eliminates the need for over provisioning and maximizes storage resource utilization facilitating database consolidation.
• Inherent large file support.
• Performs automatic online redistribution after the incremental addition or removal of storage  capacity.
• Maintains redundant copies of data to provide high availability, or leverages 3rd party RAID functionality.
• Supports Oracle Database as well as Oracle Real Application Clusters (RAC).
• Capable of leveraging 3rd party multipathing technologies.
• For simplicity and easier migration to ASM, an Oracle database can contain ASM and non-ASM files.
• Any new files can be created as ASM files whilst existing files can also be migrated to ASM.
• RMAN commands enable non-ASM managed files to be relocated to an ASM disk group.
• Enterprise Manager Database Control or Grid Control can be used to manage ASM disk and file activities.

3) Describe about ASM architecture.

Automatic Storage Management (ASM) instance

Instance that manages the diskgroup metadata

Disk Groups

Logcal grouping of disks
Determines file mirroring options
ASM Disks
LUNs presented to ASM
ASM Files
Files that are stored in ASM disk groups are called ASM files, this includes database files

Notes:

Many databases can connect as clients to single ASM instances

ASM instance name should only be +ASM only

One diskgroup can serve many databases

4) How does database connects to ASM Instance?

The database communicates with ASM instance using the ASMB (umblicus process) process. Once the database obtains the necessary extents from extent map, all database IO going  forward is processed through by the database processes, bypassing ASM. Thus we say ASM is not really in the IO path. So, the question how do we make ASM go faster…..you don’t have to.

4) What init.ora parameters does a user need to configure for ASM instances?

The default parameter settings work perfectly for ASM. The only parameters needed for 11g ASM:
• PROCESSES*
• ASM_DISKSTRING*
• ASM_DISKGROUPS
• INSTANCE_TYPE

5) How does the database interact with the ASM instance and how do I make ASM go faster?

ASM is not in the I/O path so ASM does not impede the database file access. Since the RDBMS instance is performing raw I/O, the I/O is as fast as possible.

6) Do I need to define the RDBMS FILESYSTEMIO_OPTIONS parameter when I use ASM?

No. The RDBMS does I/O directly to the raw disk devices, the FILESYSTEMIO_OPTIONS  parameter is only for filesystems.

7) Why Oracle recommends two diskgroups?

Oracle recommends two diskgroups to provide a balance of manageability, utilization, and performance.

8) We have a 16 TB database. I’m curious about the number of disk groups we should use; e.g. 1 large disk group, a couple of disk groups, or otherwise?

For VLDBs you will probably end up with different storage tiers; e.g with some of our large customers they have Tier1 (RAID10 FC), Tier2 (RAID5 FC), Tier3 (SATA), etc. Each one of these is mapped to a diskgroup.

9) We have a new app and don’t know our access pattern, but assuming mostly sequential access, what size would be a good AU fit?

For 11g ASM/RDBMS it is recommended to use 4MB ASM AU for disk groups. See Metalink Note 810484.1

10) Would it be better to use BIGFILE tablespaces, or standard tablespaces for ASM?

The use of Bigfile tablespaces has no bearing on ASM (or vice versa). In fact most database object related decisions are transparent to ASM.

11) What is the best LUN size for ASM?

There is no best size! In most cases the storage team will dictate to you based on their standardized LUN size. The ASM administrator merely has to communicate the ASM Best Practices and application  characteristics to storage folks :
• Need equally sized / performance LUNs
• Minimum of 4 LUNs
• The capacity requirement
• The workload characteristic (random r/w, sequential r/w) & any response time SLA
Using this info , and their standards, the storage folks should build a nice LUN group set for you.

12) In 11g RAC we want to separate ASM admins from DBAs and create different users and groups. How do we set this up?

For clarification
• Separate Oracle Home for ASM and RDBMS.
• RDBMS instance connects to ASM using OSDBA group of the ASM instance.
Thus, software owner for each RDBMS instance connecting to ASM must be
a member of ASM's OSDBA group.
• Choose a different OSDBA group for ASM instance (asmdba) than for
RDBMS instance (dba)
• In 11g, ASM administrator has to be member of a separate SYSASM group to
separate ASM Admin and DBAs.

13) Can my RDBMS and ASM instances run different versions?

Yes. ASM can be at a higher version or at lower version than its client databases. There’s two
components of compatiblity:
Software compatibility
Diskgroup compatibility attributes:
compatible.asm
compatible.rdbms

14) Where do I run my database listener from; i.e., ASM HOME or DB HOME?

It is recommended to run the listener from the ASM HOME. This is particularly important for RAC env, since the listener is a node-level resource. In this config, you can create additional [user] listeners from the database homes as needed.

15) How do I backup my ASM instance?

Not applicable! ASM has no files to backup, as its does not contain controlfile,redo logs etc.

16) When should I use RMAN and when should I use ASMCMD copy?

• RMAN is the recommended and most complete and flexible method to backup and transport database files in ASM.
  ASMCMD copy is good for copying single files
  • Supports all Oracle file types
  • Can be used to instantiate a Data Guard environment
  • Does not update the controlfile
  • Does not create OMF files

17) I’m going to do add disks to my ASM diskgroup, how long will this rebalance take?

• Rebalance time is heavily driven by the three items:
  1) Amount of data currently in the diskgroup
  2) IO bandwidth available on the server
  3) ASM_POWER_LIMIT or Rebalance Power Level

18) We are migrating to a new storage array. How do I move my ASM database from storage A to storage B?

Given that the new and old storage are both visible to ASM, simply add the new disks to the ASM disk group and drop the old disks. ASM rebalance will migrate data online.

Note 428681.1 covers how to move OCR/Voting disks to the new storage array

19) Is it possible to unplug an ASM disk group from one platform and plug into a server on another platform (for example, from Solaris to Linux)?

No. Cross-platform disk group migration not supported. To move datafiles between endian-ness platforms, you need to use XTTS, Datapump or Streams.

20) How does ASM work with multipathing software?

It works great! Multipathing software is at a layer lower than ASM, and thus is transparent.
You may need to adjust ASM_DISKSTRING to specify only the path to the multipathing pseudo devices.

21) Is ASM constantly rebalancing to manage “hot spots”?

No…No…Nope!! ASM provides even distribution of extents across all disks in a disk group. Since each disk will equal number of extents, no single disk will be hotter than another. Thus the answer NO, ASM does not dynamically move hot spots, because hot spots simply do not
occur in ASM configurations. Rebalance only occurs on storage configuration changes (e.g. add, drop, or resize disks).

22) What are the file types that ASM support and keep in disk groups?

Control files
Flashback logs
Data Pump dump sets

Data files
DB SPFILE
Data Guard configuration

Temporary data files
RMAN backup sets
Change tracking bitmaps

Online redo logs
RMAN data file copies
OCR files

Archive logs
Transport data files
ASM SPFILE

23. List Key benefits of ASM?

• Stripes files rather than logical volumes
• Provides redundancy on a file basis
• Enables online disk reconfiguration and dynamic rebalancing
• Reduces the time significantly to resynchronize a transient failure by tracking changes while disk is offline
• Provides adjustable rebalancing speed
• Is cluster-aware
• Supports reading from mirrored copy instead of primary copy for extended clusters
• Is automatically installed as part of the Grid Infrastructure

24. What is ASM Striping?

ASM can use variable size data extents to support larger files, reduce memory requirements, and improve performance.

Each data extent resides on an individual disk.

Data extents consist of one or more allocation units.

The data extent size is:

• Equal to AU for the first 20,000 extents (0–19999)
• Equal to 4 × AU for the next 20,000 extents (20000–39999)
• Equal to 16 × AU for extents above 40,000

ASM stripes files using extents with a coarse method for load balancing or a fine method to reduce latency.

• Coarse-grained striping is always equal to the effective AU size.
• Fine-grained striping is always equal to 128 KB.

26. How many ASM Diskgroups can be created under one ASM Instance?

ASM imposes the following limits:

• 63 disk groups in a storage system
• 10,000 ASM disks in a storage system
• Two-terabyte maximum storage for each ASM disk (non-Exadata)
• Four-petabyte maximum storage for each ASM disk (Exadata)
• 40-exabyte maximum storage for each storage system
• 1 million files for each disk group
• ASM file size limits (database limit is 128 TB):

1. External redundancy maximum file size is 140 PB.
2. Normal redundancy maximum file size is 42 PB.
3. High redundancy maximum file size is 15 PB.

27) What is a diskgroup?

A disk group consists of multiple disks and is the fundamental object that ASM manages. Each disk group contains the metadata that is required for the management of space in the disk group. The ASM instance manages the metadata about the files in a Disk Group in the same way that a file system manages metadata about its files. However, the vast majority of I/O operations do not pass through the ASM instance. In a moment we will look at how file
I/O works with respect to the ASM instance.

29) Diagram that how database interacts with ASM when a request is to read or open a datafile.

1A. Database issues open of a database file
1B. ASM sends the extent map for the file to database instance. Starting with 11g, the RDBMS only receives first 60 extents the remaining extents in the extent map are paged in on demand, providing a faster open
2A/2B. Database now reads directly from disk
3A.RDBMS foreground initiates a create tablespace for example
3B. ASM does the allocation for its essentially reserving the allocation units
for the file creation
3C. Once allocation phase is done, the extent map is sent to the RDBMS
3D. The RDBMS initialization phase kicks in. In this phase the initializes all
the reserved AUs
3E. If file creation is successful, then the RDBMS commits the file creation

Going forward all I/Os are done by the RDBMS directly

30) Can my disks in a diskgroup can be varied size? For example one disk is of 100GB and another disk is of 50GB. If so how does ASM manage the extents?

Yes, disk sizes can be varied, Oracle ASM will manage data efficiently and intelligent by placing the extents proportional to the size of the disk in the disk group, bigger diskgroups have more extents than lesser ones.

31) What is Intelligent Data Placement?

32) What is ASM preferred Mirror read? How does it useful?

33) What is ACFS?

34) What is ADVM?

Still More to add.

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