Oracle recommends a minimum of four ASM disks per ASM diskgroup. Each ASM disk should be the same size and from the same storage pool to allow ASM to properly stripe data in a manner that compliments with the ScaleIO striping.
Multiple ASM disks allows for multiple I/O queues in the OS, and so can assist with performance. However too many disks may prove cumbersome to manage.
When choosing how many disks to use in a new ASM diskgroup, one option is to determine the expected data size after the first 12 months, and then factor in the expected rate of growth.
Because all disks in a diskgroup should be of uniform size, a smaller initial disk size will provide for more granular growth. A larger disk size will result in a fewer number of disks initially and will allow for larger capacity growth without demanding an excessive number of new disks later on. However, if the DBA selects thick provisioning, larger disks may waste capacity.
Table 4 summarizes the disk size/growth calculation.
Table 4. Disk size and growth calculation
Expected rate of growth (%YOY) |
Number of disks for Year 1 |
25% or higher |
4 |
20% to 25% |
5 |
16% to 20% |
6 |
12% to 16% |
8 |
Less than 12% |
10 |
Assuming an expected 1 TB of data by the end of the first year, and an expected rate of growth of 25 percent, we might select to create our ASM diskgroup with four 250 GB volumes.
Figure 3. Capacity sizing over 60 months
Remember that the largest disk Oracle ASM is able to use is 2 TB for ASM 11gR2, and 32 PB for ASM 12c.
Note: Dell EMC recommends a minimum of four disks per ASM diskgroup.