Automatic Storage Management (ASM) - Features,Properties - Oracle Control Files

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• Nirja Shah   -Posted on 28 Aug 15
  - ASM is a volume manager and a file system for Oracle database files that supports single-instance Oracle Database and Oracle Real Application Clusters (Oracle RAC) configurations.
  - ASM is Oracle's recommended storage management solution that provides an alternative to conventional volume managers, file systems, and raw devices.
  
  - ASM provides mirroring on file by file basis - Mirroring is the simplest way to gain data redundancy to safeguard data against disk failure.
  - In mirroring, two or more (minimum of three in LVM) disks or partitions are combined and identical data is written to each device.
  - The space available is only as large as the smallest device in the mirror.
  - Some implementations of mirroring offer increased read performance, but write performance is typically similar to traditional disk performance.
  - What mirroring does offer is the ability to keep running or recover more quickly as a redundant copy of the data is kept online and up-to-date.
  
  - ASM provides automatic load balancing across all ASM disks - Oracle has been promoting the concept of SAME.
  - Stripe and Mirror Everything which aimed at using the full I/O capacity of the all disks. Many database users have implemented SAME sets, each of about 4 mirrored disk pairs.
  - ASM takes an additional step in balancing I/O activity.
  - Rebalancing distributes file data evenly across all the disks of the ASM Disk Group.
  - ASM automatically rebalances a disk group whenever disks are added or dropped.
  - ASM ensures that a file is evenly spread across all disks in a disk group when the file is allocated, so rebalancing is not required except when the storage configuration changes.
  - With the I/O balanced whenever files are allocated and whenever the storage configuration changes, there is no need to search for hot spots in a disk group and manually move data to restore a balanced I/O load.
  - It also allows the addition of disks online.
  - There are extra background processes that conduct the rebalancing activity in a non-intrusive way.
  - Thus, ASM makes manual load balancing unnecessary because the dynamic rebalancing overcomes the major drawback of SAME.
  - When new disks are added to a disk group, rebalancing happens automatically without an outage.
  
  - The other two statements which are not true about ASM are:
  - ASM supports the Oracle database and operating system files
  - ASM can be used to store trace files, alert log files, and the server parameter file (SPFILE)

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