Understanding Oracle ASM Striping

Oracle ASM striping has two primary purposes:

• To balance loads across all of the disks in a disk group

• To reduce I/O latency

Coarse-grained striping provides load balancing for disk groups while fine-grained striping reduces latency for certain file types by spreading the load more widely.

To stripe data, Oracle ASM separates files into stripes and spreads data evenly across all of the disks in a disk group. The fine-grained stripe size always equals 128 KB in any configuration; this provides lower I/O latency for small I/O operations. The coarse-grained stripe size is always equal to the AU size (not the data extent size).

Figure 1-5 and Figure 1-6 are illustrations of Oracle ASM file striping. In both illustrations, the allocation unit size has been set to 1 M (AU_SIZE = 1M) for the disk group which consists of 8 disks. The instance is Oracle ASM 11g Release 2 (11.2) and the disk group compatibility attributes for ASM and RDBMS have been set to 11.2, so variable extents are shown in the graphic after the first 20,000 extents. For the first 20,000 extents, the extent size is 1 M and equals one allocation unit (AU). For the next 20,000 extents, the extent size is 4 M and equals 4 AUs.

To identify the stripe chunks of the file, they have been labeled A..X (24 letters) using different fonts for successive series of A..X until all the chunks have been identified.

In Figure 1-5, the file is striped in 128 K chunks (labeled A..X) with each 128 K chunk stored in an extent, starting at the first extent in disk 1, then the first extent in disk 2, and then continuing in a round-robin pattern through all the disks until the entire file has been striped. As shown in this example, the striping chunks first fill up the first extent of each disk, then the second extent of each disk, and so on until the entire file has been striped.

In Figure 1-6, the file is striped in 1 M chunks (labeled A..X) with each 1 M chunk stored uniquely in an extent, starting at the first extent in disk 1, then the first extent in disk 2, and then continuing in a round-robin pattern through all the disks until the entire file has been striped. For the first 20,000 extents where the AU equals the extent size (1 M), the stripe equals the extent size and allocation unit size.For the variable extents, where an extent is composed of multiple allocation units, the file stripe is located in an AU of the extent. The striping chunks are placed in the allocation units of the first extents of all the disks before the striping continues to the next extent.