Understanding Oracle ASM Extents

The contents of Oracle ASM files are stored in a disk group as a set, or collection, of extents that are stored on individual disks within disk groups. Each extent resides on an individual disk. Extents consist of one or more allocation units (AU). To accommodate increasingly larger files, Oracle ASM uses variable size extents.

Variable size extents enable support for larger Oracle ASM data files, reduce SGA memory requirements for very large databases, and improve performance for file create and open operations. The initial extent size equals the disk group allocation unit size and it increases by a factor of 4 or 16 at predefined thresholds. The various extent sizes are described in this section.

For disk groups with AU size less than 4 MB:

• Extent size always equals the disk group AU size for the first 20000 extent sets (0 - 19999).

• Extent size equals 4*AU size for the next 20000 extent sets (20000 - 39999).

• Extent size equals 16*AU size for the next 20000 and higher extent sets (40000+).

For disk groups with AU size greater than or equal to 4 MB and the disk group RDBMS compatibility greater than or equal to 11.2.0.4, the counts for extents of sizes (the disk group AU size, 4*AU size, or 16*AU size) are calculated using the application block size to support maximum file size.

The extent sizing feature is automatic for newly created and resized data files when specific disk group compatibility attributes are set to 11.1 or higher. For information about compatibility attributes, see "Disk Group Compatibility".

Figure 1-4 shows the Oracle ASM file extent relationship with allocation units. The first eight extents (0 to 7) are distributed on four Oracle ASM disks and are equal to the AU size. After the first 20000 extent sets, the extent size becomes 4*AU for the next 20000 extent sets (20000 - 39999). This is shown as bold rectangles labeled with the extent set numbers 20000 to 20007, and so on. The next increment for an Oracle ASM extent is 16*AU (not shown in Figure 1-4).