Units about ASM

1.ASM Striping and Mirroring:ASM supports two levels of striping: fine striping and coarse striping. Fine striping uses 128KB as the stripe width, and coarse striping uses 1MB as the stripe width.

2.ASM Allocation Unit:ASM disks are divided into a number of units or storage blocks that are small enough not to be hot. The allocation unit defaults to 1MB in size and is sufficient for most configurations. ASM allows you to change the allocation unit size, You cannot change the size of an allocation unit of an existing disk group.

3.ASM Volume Allocation Unit:Like the allocation unit of the ASM disk group, the volume allocation unit is the smallest storage unit that ASM allocates for space inside the ASM dynamic volume. The size of the volume allocation unit is related to the size of the allocation unit of the ASM disk group. By default, ASM creates a volume allocation unit of 64MB inside the ASM disk group created with a default allocation unit size of 1MB.

4.ASM file extent:An ASM file is a collection of data extents, where each data extent is a collection of allocation units. You cannot change the data extent size because Oracle will automatically increase the size of the data extent when an ASM file increases in size.First, 20,000 data extents are allocated as one allocation unit per extent and thereafter the data extent size for the next 20,000 data extents is eight allocation units. When the ASM file is larger than 40,000 data extents, the data extent size for data extents above 40,000 is 64 allocation units per extent.