Is it mandatory to use disks of same size and characteristics for Diskgroups?
No, it is not mandatory to use the disks of same size and characteristics for Diskgroups though it is a Recommended Practice. Same size disk for Failuregroups in Normal/High redundancy will prevent issues like ORA-15041 as the file extents needs to be mirrored across the disks. Also as Oracle distributes data based on capacity, so larger disk will have more data stored in it and which will result in higher I/O to disk and eventually can lead to sub-optimal performance. Moreover having disks of different characteristic like varying disk speed can impact the performance. When managing disks with different size and performance capabilities, best practice is to group them into disk groups according to their characteristics. So you can use higher speed disks for your database files while other disks can be part of Diskgroup used for Flash Recovery Area.
