ASM Normal Redundancy versus High Redundancy on Oracle Database Appliance

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*)

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).

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Comments ( 1 )
  • AMIT KUMAR AGNIHOTRI Friday, June 20, 2014

    Excellent quick review about mirroring...

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