ASM DISKS – DISKGROUPS

Automatic Storage Management (ASM) is an integrated file system and volume manager expressly built for Oracle database files. ASM provides the performance of raw I/O with the easy management of a file system. It simplifies database administration by eliminating the need for you to directly manage potentially thousands of Oracle database files. It does this by enabling you to divide all available storage into disk groups. You manage a small set of disk groups and ASM automates the placement of the database files within those disk groups.

Disk Groups

The primary component of ASM is the disk group. A disk group consists of a grouping of disks that are managed together as a unit. You configure ASM by creating disk groups to store database files. Oracle provides SQL statements that create and manage disk groups, their contents, and their metadata.

Disks

The disks in a disk group are referred to as ASM disks. On Windows operating systems, an ASM disk is always a partition. On all other platforms, an ASM disk can be

A partition of a logical unit number (LUN)

A network-attached file

Note: Although you can also present a volume (a logical collection of disks) for management by ASM, it is not recommended to run ASM on top of another host-based volume manager.

When an ASM instance starts, it automatically discovers all available ASM disks. Discovery is the process of determining every disk device to which the ASM instance has been given I/O permissions (by some operating system mechanism), and of examining the contents of the first block of such disks to see if they are recognized as belonging to a disk group. 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.

Discovery Rules

· ASM discovers no more than 10,000 disks. That is, if more than 10,000 disks match the ASM_DISKSTRING initialization parameter, only the first 10,000 are discovered.

· ASM does not discover a disk that contains an operating system partition table, even if the disk is in an ASM_DISKSTRING search path and ASM has read/write permission on the disk.

· If ASM recognizes a disk header as that of an Oracle object, such as the header of an Oracle datafile, the disk is discovered, but can only be added to a disk group with the FORCE keyword. Such a disk appears in V$ASM_DISK with a header status of FOREIGN.

In addition, ASM identifies the following configuration errors during discovery:

· Multiple paths to the same disk : In this case, if the disk is part of a disk group, disk group mount fails. If the disk is being added to a disk group with the ADD DISK or CREATE DISKGROUP command, the command fails. To correct the error, restrict ASM_DISKSTRING so that it does not include multiple paths to the same disk, or if you are using multipathing software, ensure that you include only the pseudo-device in ASM_DISKSTRING.

· Multiple ASM disks with the same disk header: This can be caused by a bit copy of one disk onto another. In this case, disk group mount fails.

Determining the Number of Disk Groups

Disks in a given disk group should have similar size and performance characteristics. If you have several different types of disks in terms of size and performance, then it would be better to form several disk groups accordingly.

For recovery reasons, you might feel more comfortable having separate disk groups for your database files and flash recovery area files. Using this approach, even with the loss of one disk group, the database would still be intact.

How ASM Handles Disk Failures

Disk failure in this context means individual spindle failure or failure of another disk subsystem component, such as power supply, a controller, or host bus adapter. Here are the rules for how ASM handles disk failures:

A failure group is considered to have failed if at least one disk in the failure group fails.
A normal redundancy disk group can tolerate the failure of one failure group. If only one failure group fails, the disk group remains mounted and serviceable, and ASM performs a rebalance of the surviving disks (including the surviving disks in the failed failure group) to restore redundancy for the data in the failed disks. If more than one failure group fails, ASM dismounts the disk group.
A high redundancy disk group can tolerate the failure of two failure groups. If one or two failure groups fail, the disk group remains mounted and serviceable, and ASM performs a rebalance of the surviving disks to restore redundancy for the data in the failed disks. If more than two failure groups fail, ASM dismounts the disk group.
An external redundancy disk group cannot tolerate the failure of any disks in the disk group. Any kind of disk failure causes ASM to dismount the disk group.