The Zero Dataloss Myth

In previous posts I have focused on the technical side of running business applications (except my last post about the Joint Escalation Center). So let’s teleport to another level and have a look at business drivers.

What happens if you are an IT architect for an organization, and you ask your business people (your internal customers) how much data loss they can tolerate in case of a disaster? I bet the answer is always the same:


This relates to what is known in the industry as Recovery Point Objective (RPO).

Ask them how much downtime they can tolerate in case something bad happens. Again, the consistent answer:


This is equivalent to Recovery Time Objective (RTO).

Now if you are in “Jukebox mode” (business asks, you provide, no questions asked) then you try to give them what they ask for (RPO = zero, RTO = zero). Which makes many IT vendors and communication service providers happy, because this means you have to run expensive clustering software, and synchronous data mirroring to a D/R site using pricey data connections.

If you are in “Consultative” mode, you try to figure out what the business really wants, not just what they ask for. And you wonder if their request is feasible at all, and if so, what the cost is of achieving these service levels.

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Exadata Hybrid Columnar Compression (HCC) for (storage) dummies

Columnar Basalt Landscape

Although EMC and Oracle have been long-time partners, the Exadata Database Machine is the exception to the rule and competes with EMC products directly. So I find myself more and more in situations where EMC offerings are compared directly with Exadata features and functions. Note that Oracle offers more competing products, including some storage offerings such as the ZFS storage appliance and the Axiom storage systems, but so far I haven’t seen a lot of pressure from those (except when these are bundled with Exadata).
Recently I have visited customers who asked me questions on how EMC technology for databases compares with, in particular, Oracle’s Hybrid Columnar Compression (HCC) on Exadata. And some of my colleagues, being storage aliens and typically not database experts, have been asking me what this Hybrid Compression thing is in the first place.

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Data Guard protecting from EMC block corruptions?

Today I was giving a training to fellow EMC colleagues on some Oracle fundamentals. One of the things that was mentioned is something I have heard several times before: Oracle is claiming that EMC SRDF (a data mirroring function from EMC Symmetrix enterprise storage systems mainly to provide enterprise disaster recovery functions) cannot detect certain types of data corruption where Oracle Data Guard can. Ouch. The trouble with this statement is that it is half-true (and these ones are the most dangerous).
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Oracle and Data Integrity: Data in, Garbage Out?

Stop Corruption

A trivial question:

What is the basic function of a storage system?
I would say, the trivial function of a storage system is to store digital data and getting it back when you need it.

To be specific:
get the data back exactly the way you stored it.

You would probably say “Duh, of course!”

A storage system (as simple as a hard disk or as sophisticated as an EMC VMAX) is supposed to store data and give it back unmodified. But recent research shows that simple disk drives are not as reliable as you might think. Enough material is available that explains why and how often disk drives fail to return the correct information, often without any error as if the corrupted data is perfectly valid. See below for more references to this issue.

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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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Data Guard or Storage based replication?

A comparison between Oracle (Active) Data Guard and EMC replication for disaster recovery purposes

Panic Button
This is an article I wrote a while ago for customers’ Database Administrators (DBAs) and application managers, that helps them in selecting the right Disaster Recovery tools for their business applications.
It is slightly modified to update new insights and to make it more readable on the web.

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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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Duplexing Oracle Redo logs?

A customer asked me recently what EMC’s advice is regarding duplexing Oracle redo logs. There is a thought behind this – Oracle redo logs are sensitive to data corruption – if redo logs are corrupt, there is no way to nicely recover the database to a technically consistent state (at least not without restoring data from backups).

This is what Oracle tells you:

Oracle recommends that you multiplex your redo log files; the loss of the log file information can be catastrophic if a recovery operation is required.


Duplexed Redo

Duplexed Redo

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