Oracle ASM Concepts

Note: Proofread any scripts before using. Always try scripts on a test instance first. This Blog is not responsible for any damage.

Oracle Automatic Storage Management - Concepts

• Oracle ASM is a volume manager and a file system for Oracle database files.
• It supports single-instance and Oracle RAC configurations.
• Oracle ASM also supports a general purpose file system that can store application files and oracle database binaries.
• It provides an alternative to conventional volume managers, file systems and raw devices.

• Oracle ASM distributes I/O load across all available resource to optimize performance.
• In this way, it removes the need for manual I/O tuning (spreading out the database files avoids hotspots).
• Oracle ASM allows the DBA to define a pool of storage (disk groups).
• The Oracle kernel manages the file naming and placement of the database files on the storage pool.

Disk groups

• Oracle ASM store data files on disk groups.
• A disk group is a collection of disks managed as a unit by Oracle ASM.
• Oracle ASM disks can be defined on:
• A disk partition: Entire disk or a section of disk that does not include the partition table (or it will be overwritten).
• A Disk from a storage array (RAID): RAID present disks as Logical Unit Numbers (LUNs).
• A logical volume.
• A Network-attached file (NFS): Including files provided through Oracle Direct NFS (dNFS).Whole disks, partitions and LUNs can also be mounted by ASM through NFS.
• Load balance: Oracle ASM spreads the files proportionally across all of the disks in the disk group, so the disks within a disk group should be in different physical drives.

Disks can be added or removed "on the fly" to and from disk groups.
After you add a disk, Oracle ASM performs rebalancing.
Data is redistributed to ensure that every file is evenly spread across all of the disks.

• Disks can be added or removed from a disk group while the database is accessing files on that disk group (without downtime).
• Oracle ASM redistributes contents automatically
• Oracle ASM uses Oracle Managed Files (OMF).

Any Oracle ASM file is completely contained within a single disk group. However, a disk group might contain files belonging to several databases. A single database can use files from multiple disk groups.

Mirroring and Failure groups

• Disk groups can be configured with varying redundancy levels.
• For each disk in a disk group, you need to specify a failure group to which the disk will belong.
• A failure group is a subset of the disks in a disk group, which could fail at the same time because they share hardware
• Failure groups are used to store mirror copies of data.
• In a normal redundancy file, Oracle ASM allocates a primary copy and a secondary copy in disks belonging to different failure groups.
• Each copy is on a disk in a different failure group so that the simultaneous failure of all disks in a failure group does not result in data loss.
• A normal redundancy disk group must contain at least two failure groups.
• Splitting the various disks in a disk group across failure groups allows Oracle ASM to implment file mirroring.
• Oracle ASM implements mirroring by allocating file and file copies to different failure groups.
• If you do not explicitly identify failure groups, Oracle allocates each disk in a disk group to its own failure group.

Oracle ASM implements one of three redundancy levels: External redundancy: No ASM mirroring. Useful when the disk group contain RAID devices Normal redundancy Oracle ASM implements 2-way mirroring by default. At least 2 failure groups are needed. Minimum of two disks in group. High redundancy Oracle ASM implements 3-way mirroring: Minimum of 3 disks in group

(a) Diskgr1 below implements 2-way mirroring. Each disk (dasm-d1, dasm-d2) is assigned to its own failure group. SQL> Create diskgroup diskgr1 NORMAL redundancy 2 FAILGROUP controller1 DISK 4 '/devices/diska1' NAME dasm-d1, 3 FAILGROUP controller2 DISK 5 '/devices/diskb1' NAME dasm-d2 6 ATTRIBUTE 'au_size'='4M';

An Oracle ASM disk is divided into allocation units (AU). Files within an ASM disk consist of one or more allocation units. Each ASM file has one or more extents. Extent size is not fixed: starting with one allocation unit, extent size increases as total file size increases.

Oracle ASM Instance

Oracle ASM metadata: disks belonging to a disk group space available in a disk group names of files in a disk group location of disk group data extents redo log for changes in metadata blocks Oracle ADVM (ASM Dynamic volume Manager) volume information

• With Oracle ASM an ASM instance besides the database instance needs to be configured on the server.
• An Oracle ASM instance has an SGA and background processes, but is usually much smaller than a database instance.
• It has minimal (how much?) performance effect on a server.
• Oracle ASM Instances are responsible for mounting the disk groups so that ASM files are available for DB instances.
• Oracle ASM instances DO NOT mount databases.
• They only manage the metadata of the disk group and provide file layout information to the database instances.

ASM Instance on Clusetered configurations:

• One Oracle ASM instance in each cluster node.
• All database instances in a node share the same ASM instance
• In a Oracle RAC environment, the ASM and database instances on the surviving nodes automatically recover from an ASM Instance failure on a node.