RoadToMaster – Automatic Storage Management (ASM) is an integrated, high-performance database file system and disk manager. ASM is based on the principle that the database should manage storage instead of requiring an administrator to do it. ASM eliminates the need for you to directly manage potentially thousands of database files.
ASM groups the disks in your storage system into one or more disk groups. You manage a small set of disk groups and ASM automates the placement of the database files within those disk groups.
ASM provides the following benefits:
■ Striping— ASM spreads data evenly across all disks in a disk group to optimize performance and utilization. This even distribution of database files eliminates the need for regular monitoring and I/O performance tuning.
For example, if there are six disks in a disk group, pieces of each ASM file are written to all six disks. These pieces come in 1 MB chunks known as extents. When a database file is created, it is striped (divided into extents and distributed) across the six disks, and allocated disk space on all six disks grows evenly. When reading the file, file extents are read from all six disks in parallel, greatly increasing performance.
■ Mirroring— ASM can increase availability by optionally mirroring any file. ASM mirrors at the file level, unlike operating system mirroring, which mirrors at the disk level. Mirroring means keeping redundant copies, or mirrored copies, of each extent of the file, to help avoid data loss caused by disk failures. The mirrored copy of each file extent is always kept on a different disk from the original copy. If a disk fails, ASM can continue to access affected files by accessing mirrored copies on the surviving disks in the disk group.
ASM supports 2-way mirroring, where each file extent gets one mirrored copy, and 3-way mirroring, where each file extent gets two mirrored copies.
■ Online storage reconfiguration and dynamic rebalancing—ASM permits you to add or remove disks from your disk storage system while the database is operating. When you add a disk, ASM automatically redistributes the data so that it is evenly spread across all disks in the disk group, including the new disk. This redistribution is known as rebalancing. It is done in the background and with minimal impact to database performance. When you request to remove a disk, ASM first rebalances by evenly relocating all file extents from the disk being removed to the other disks in the disk group.
■ Managed file creation and deletion—ASM further reduces administration tasks by enabling files stored in ASM disk groups to be Oracle-managed files. ASM automatically assigns filenames when files are created, and automatically deletes files when they are no longer needed.
Oracle recommends that you use ASM for your database file storage, instead of raw devices or the operating system file system. However, databases can have a mixture of ASM files and non-ASM files. Oracle Enterprise Manager includes a wizard that enables you to migrate non-ASM database files to ASM.
The ASM Instance
ASM is implemented as a special kind of Oracle instance, with its own System Global Area and background processes. The ASM instance is tightly integrated with the database instance. Every server running one or more database instances that use ASM for storage has an ASM instance. In a Real Application Clusters environment, there is one ASM instance for each node, and the ASM instances communicate with each other
on a peer-to-peer basis. Only one ASM instance is required for each node regardless of the number of database instances on the node.
Overview of Disks, Disk Groups, and Failure Groups
You configure ASM by creating disk groups that become the default location for files created in the database. The disk group type determines how ASM mirrors files. When you create a disk group, you indicate whether the disk group is a normal redundancy, disk group (2-way mirroring for most files by default), a high redundancy disk group (3-way mirroring), or an external redundancy disk group (no mirroring by ASM). You use an external redundancy disk group if your storage system already does mirroring at the hardware level, or if you have no need for redundant data. The default disk group type is normal redundancy.
A disk group consists of a grouping of disks that are managed together as a unit. These disks are referred to as ASM disks. An ASM disk can be a disk device, a partition, or a network-attached file.
When an ASM instance starts, it automatically discovers all available ASM disks. Discovery is the process of finding all disks that have been prepared for ASM by your system administrator, examining their disk headers, and determining which disks belong to disk groups and which are available for assignment to disk groups. ASM discovers disks in the paths that are listed in an initialization parameter, or if the parameter is NULL, in an operating system–dependent default path.
Failure groups define ASM disks that share a common potential failure mechanism. An example of a failure group is a set of SCSI disks sharing the same SCSI controller. Failure groups are used to determine which ASM disks to use for storing redundant copies of data. For example, if 2-way mirroring is specified for a file, ASM automatically stores redundant copies of file extents in separate failure groups. Failure groups apply only to normal and high redundancy disk groups. You define the failure groups in a disk group when you create or alter the disk group.
By default, when you create a disk group, every disk in the disk group belongs to exactly one failure group. For most systems, the default failure groups work well to prevent data loss.
DBA In Seconds 