POC: Piece Of Cake or Point Of Contradiction?

Every now and then I get involved in Customer Proof of Concepts. A Proof of Concept (POC) is, according to Wikipedia, something like a demonstration of feasibility of a certain idea, concept or theory.

Concept Performance Aircraft

Concept Aircraft

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Oracle Stretched Cluster with VPLEX (update)

One request I got back after my series on Oracle RAC stretched clusters is if I could summarize again why anybody would choose VPLEX for storage replication over other solutions. My attempt was to describe the principles of VPLEX in enough detail for techies to understand it. For non-geeks, I will try to explain it as brief as possible.
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Through the wormhole with Stretched Clusters

Last year, EMC announced a new virtualization product called VPLEX. VPLEX allows logical storage volumes to be accessible from multiple locations. It boldly goes beyond existing storage virtualisation solutions (including those from EMC) in that it is not just a storage virtualisation cluster – but merely a storage federation platform, allowing one virtualized storage volume to be dynamically accessible from multiple locations, as if they were connected through a wormhole, and being built from one or more physical storage volumes.

Wormhole in space
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Stretched Clusters – Alien storage

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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Limitations of host-based mirroring for stretched clusters

For data mirroring, EMC SRDF is sometimes used in such a setup that both servers write to one location only (the “far” server writes across dark fibre links to the local storage). EMC has similar tools (Mirrorview, Recoverpoint, etc) for other storage platforms than Symmetrix.

srdf cluster

SRDF cluster with passive target

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Extreme availability with Oracle stretched clusters

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.


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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