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A simple representation of redundancy

on Saturday, 12 February 2011. Posted in General

The term redundancy is often misinterpreted with availability. While these two are related, they are not the same. Redundancy refers to as example; the use of multiple servers, more than one Host Bus Adapter (HBA) or RAID protected disks. Redundancy is the duplication of critical components of a system with the intention of increasing reliability of the system. When it comes to redundancies, the more is better you would say. But redundancy has its price. Why? Because you have to buy at least double the hardware. On the other hand, what is the price of a server compared to the outage of your mission critical application?
There are many scenarios to deploy a redundant solution for servers and storage. Let’s look at some simplified examples:


No redundancy:

Let’s look at the components in the picture. You only see a single points of failure. A single point of failure (SPOF) is a hardware or software element whose loss results in the loss of service.


HBA redundancy:

Configuring multiple HBAs and using multi-pathing software provides path redundancy. Upon detection of one failed HBA, the software can re-drive the I/O through another available path. You can also cross redundancy with 4 pairs which is not shown in the picture.


HBA and Switch redundancy:

This picture provides HBA and switch redundancy as well. It also protects against storage array port failures. Same here, cross redundancy with 4 pairs which is not shown in the picture. It applies for the other pictures as well.


HBA, Switch and Disk redundancy:

Now we are using some level of RAID such as RAID-5. Raid protection will ensure continuous operation in the event of disk failures.


HBA, Switch, Disk and Storage array redundancy:

Looking at this we see a high level redundancy infrastructure. Everything is redundant and there is little chance that if one component breaks down your applications are no longer available.
Remote replication is an essential part of any data protection plan. It provides protection in case of primary device, storage or site failure. Remote replication involves moving data to a secondary storage array. Remote replication protects against data loss in case of primary site failure.
There are two types of remote replication, synchronous, which allows RPO of close to zero and A-synchronous which allows updates to be made to a Secondary Image at intervals selected by the user.
Bottom line is, there should be any kind of redundancy in your infrastructure to make sure all data and information is protected. Whiteout redundancy you will see when an outage happens, and believe me it will happen, you are lost. You will have to take a decision how far you want to pull redundancy. As mentioned earlier this also has a price. The more redundancy the more money it costs.

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