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-d1
  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.

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