About ASM Allocation Units, Extents, Mirroring and Failgroups

ASM Allocation Units

An ASM allocation unit (AU) is the fundamental space unit within an ASM disk group. Every ASM disk is divided into allocation units.

When a disk group is created, the allocation unit size can be set with the  disk group attribute AU_SIZE (in ASM versions 11.1 and later). The AU size can be 1, 2, 4, 8, 16, 32 or 64 MB. If not explicitly set, the AU size defaults to 1 MB (4MB in Exadata).

AU size is a disk group attribute, so each disk group can have a different AU size.

ASM Extents

An ASM extent consists of one or more allocation units. An ASM file consists of one or more ASM extents.

We distinguish between physical and virtual extents. A virtual extent, or an extent set, consists of one physical extent in an external redundancy disk group, at least two physical extents in a normal redundancy disk group and at least three physical extents in a high redundancy disk group.

Before ASM version 11.1 we had uniform extent size. ASM version 11.1 introduced the variable sized extents that enable support for larger data files, reduce (ASM and database) SGA memory requirements for very large databases, and improve performance for file create and open operations. The initial extent size equals the disk group AU_SIZE and it increases by a factor of 4 or 16 at predefined thresholds. This feature is automatic for newly created and resized data files with disk group compatibility attributes COMPATIBLE.ASM and COMPATIBLE.RDBMS set to 11.1 or higher.

The extent size of a file varies as follows:

• Extent size always equals the disk group AU_SIZE for the first 20,000 extent sets
• Extent size equals 4*AU_SIZE for the next 20,000 extent sets
• Extent size equals 16*AU_SIZE for the next 20,000 and higher extent sets

There is nasty bug 8898852 to do with this feature. See more on that in MOS Doc ID 965751.1.

ASM Mirroring

ASM mirroring protects data integrity by storing multiple copies of the same data on different disks. When a disk group is created, ASM administrator can specify the disk group redundancy as follows:

• External – no ASM mirroring
• Normal – 2-way mirroring
• High – 3-way mirroring

ASM mirrors extents – it does not mirror disks or blocks. ASM file mirroring is the result of mirroring of the extents that constitute the file. In ASM we can specify the redundancy level per file. For example, one file in a normal redundancy disk group, can have its extents mirrored once (default behavior). Another file, in the same disk group, can be triple mirrored – provided there are at least three failgroups in the disk group.  In fact all ASM metadata files are triple mirrored in a normal redundancy disk group – provided there are at least three failgroups.

ASM Failgroups

ASM disks within a disk group are partitioned into failgroups (also referred to as failure groups or fail groups). The failgroups are defined at the time the disk group is created.  If we omit the failgroup specification, then ASM automatically places each disk into its own failgroup. The only exception is Exadata, where all disks from the same storage cell are automatically placed in the same failgroup.

Normal redundancy disk groups require at least two failgroups. High redundancy disk groups require at least three failgroups. Disk groups with external redundancy do not have failgroups.

When an extent is allocated for a mirrored file, ASM allocates a primary copy and a mirror copy. Primary copy is store on one disk and the mirror copy on some other disk in a different failgroup.

When adding disks to an ASM disk group for which failgroups are manually specified, it is imperative to add the disks to the correct failgroup.