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The Allocation Unit or AU size is the smallest amount of storage that may be allocated to or removed from an ASM diskgroup.
The AU size was set at the ASM instance level via the hidden parameter _asm_ausize.
The AU when a diskgroup is created, but once created it cannot be altered, so it is important to set this correctly before data is loaded into ASM.
The diskgroup AU size still defaults to the value specified in the hidden _asm_ausize parameter. The ASM AU size may be specified as 1 MB, 2 MB, 4 MB, 8 MB, 16 MB, 32 MB or 64 MB.
The AU size governs the maximum size of the disks that may be added to a diskgroup, as shown in Table 6:
Table 6. AU and ASM disk size
AU size |
Maximum ASM disk size |
1 MB |
4 PB |
2 MB |
8 PB |
4 MB |
16 PB |
8 MB and above |
32 PB |
The following example shows a new diskgroup called BIG_DATA being created on a Linux system with ASMLib with a 16 MB allocation unit:
SQL> create diskgroup BIG_DATA external redundancy disk '/dev/oracleasm/bigdata01'
2 attribute 'au_size' = '16M';
Diskgroup created.
The allocation unit does not directly govern the size of I/O to the storage subsystem that is controlled by the extent size or the maximum ASM I/O size parameter which will be covered later.
However, allocation unit size does control the size of the metadata held in ASM, and for larger databases there is a performance advantage of using a larger AU size.
A 1 GB data file using the default Allocation Unit would require 1024 1 MB extents. Setting the AU to 4 MB would reduce the extent count to 256.
Furthermore, due to ASM managing data in ASM extents which are derived from the allocation unit, a larger AU size increases the likelihood that data will be stored sequentially on spinning disk.
In our example above, with 1 GB of data, the 1024 1 MB extents may be stored randomly across the available disks as ASM seeks to balance the capacity of each disk.
The 256 4 MB extents will also be stored randomly, but during any scanning of the data there will be far fewer extents of data to be randomly retrieved. Data is more likely to occur sequentially and seek times are significantly reduced where spinning disks are used.
Testing has shown that large data warehouses can benefit from up to 16 percent performance improvement using a larger AU size of 8 MB or 16 MB.
Dell EMC recommends an AU size of between 1 MB and 4 MB for mostly OLTP workloads, and an 8 MB or larger AU size for OLAP workloads.
For very large databases, a 16 MB or larger AU size should be considered for diskgroups where the majority of the data will be stored.