ASM Normal Redundancy versus High Redundancy on Oracle Database Appliance
By Ravi.Sharma on Dec 21, 2012
The availability of normal redundancy configuration option for Automatic Storage Management (ASM) on Oracle Database Appliance starting with OAK version 2.4 allows for additional usable space on Oracle Database Appliance (about 6 TB with Normal Redundancy versus about 4 TB with High Redundancy). This is great news for many customers. Some environments, such as test and development systems, may benefit significantly as a result of this new option. However, the availability of Normal Redundancy option obviously should not be taken to mean that choosing Normal Redundancy may the best approach for all database environments. High redundancy would still provide a better and more resilient option (and may be a preferred choice) for mission critical production systems. It is therefore an option and not the default configuration choice. Many customers may choose to use Normal Redundancy for test, development, and other non-critical environments and High Redundancy for production and other important systems.
In general, ASM supports three types of redundancy (mirroring*) options.
High Redundancy - In this configuration, for each primary extent, there are two mirrored extents. For Oracle Database Appliance this means, during normal operations there would be three extents (one primary and two secondary) containing the same data, thus providing “high” level of protection. Since ASM distributes the partnering extents in a way that prevents all extents to be unable due to a component failure in the IO path, this configuration can sustain at least two simultaneous disk failures on Oracle Database Appliance (which should be rare but is possible).
Normal Redundancy - In this configuration, for each primary extent, there is one mirrored (secondary) extent. This configuration protects against at least one disk failure. Note that in the event a disk fails in this configuration, although there is typically no outage or data loss, the system operates in a vulnerable state, should a second disk fail while the old failed disk replacement has not completed. Many Oracle Database Appliance customers thus prefer the High Redundancy configuration to mitigate the lack of additional protection during this time.
External Redundancy - In this configuration there are only primary extents and no mirrored extents. This option is typically used in traditional non-appliance environments when the storage sub-system may have existing redundancy such as hardware mirroring or other types of third-party mirroring in place. Oracle Database Appliance does not support External Redundancy.
*ASM redundancy is different from traditional disk mirroring in that ASM mirroring is a logical-physical approach than a pure physical approach. ASM does not mirror entire disks. It mirrors logical storage entities called ‘extents’ that are allocated on physical disks. Thus, all “mirrored” extents of a set of primary extents on a given disk do not need to be on a single mirrored disk but they could be distributed across multiple disks. This approach to mirroring provides significant benefits and flexibility. ASM uses intelligent, Oracle Database Appliance architecture aware, extent placement algorithms to maximize system availability in the event of disk failure(s).