Stretched Clusters – Alien storage

Bart Sjerps Oracle, VPLEX No Comments

In my previous posts I described how Oracle ASM can be used to build stretched clusters. I also pointed to some limitations of that scenario. But I am by far not the first one in doing so – and some of EMC’s competitors attempted to build products, features and solutions to overcome some of the limitations in host mirroring.

A while ago, some guys I met from an EMC partner, confronted me with the question why EMC, the market leader in external storage and premium Oracle technology partner, had not offered a solution for these limitations. They pointed to a number of products from competitors that – allegedly – solved the problem already. Also they pointed to the architectural simplicity of these solutions.

Alien Storage

At that time I had no good answer (which does not happen to me very often). I was not aware of how these products worked and I asked some questions on that. In that period I was also confronted by our enterprise customers who started demanding an EMC solution for stretched clustering – so I started digging. Could it be that EMC was over-passed by some of these alien storage start-up companies in continuous available storage solutions? It seemed to be the case.
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Stretched clustering basics

Bart Sjerps Oracle, VPLEX No Comments

Before showing my preferred solution for Oracle stretched high availability clusters, first some clustering basics.

Active/passive versus Active/Active clusters

NASA cluster
NASA cluster

Most clustering software is based on active/passive scenarios. You have a system (say, a database) that is running on a set of resources (i.e. a server, to keep it simple) and you have another system (a standby system) that is ready to run the system (failover) but is not actually running it at the same time.

An Active/Active system in general means both systems are active at the same time. There is some confusion as this can mean that the standby system is used for other processing (say, A is production and B is standby but currently running acceptance or testing environments).

By my definition, active/active clustering describes a cluster where all cluster nodes (systems) are processing against the same data set at the same time. There aren’t many products that can do this, especially in the database world. One of the few exceptions is Oracle RAC.

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Extreme availability with Oracle stretched clusters

Bart Sjerps Oracle, VPLEX No Comments

Some of my customers have been pushing for more availability in their Oracle database applications. They want to eliminate downtime completely even if they experience a site failure. Whether this is a real business requirement or a technology push, I’m not sure – I guess a bit of both.

ha_aircraft

Most of these customers have already implemented Oracle RAC (Real Application Clusters), which provides them active/active server clustering for Oracle. If one of the servers in a RAC cluster fails, the others just keep running – no restart or recovery involved. This is a High Availability option typically for local sites.

For Disaster Recovery, most customers have some sort of storage replication (i.e. EMC SRDF/Synchronous or SRDF/Async, or they use Oracle Data Guard for this which replicates data on the Oracle database level). This protects against site failures and offers zero or near-zero dataloss (for committed transactions in Oracle – the non-committed transactions are rolled back during the restart – and this is exactly one of the problems by the way).
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