Why would you not want to consolidate Mission Critical Databases?

Suppose if you wanted to consolidate all of your mission critical databases into one instance so that you can save some licensing money, what would be the potential risks and are there any good articles or case studies on this? I realize this is a terrible idea but I have somebody that wants to do this and is willing to maximize the hardware resources if needed. I am trying to present him with something quantifiable or some articles that can steer him away from doing this.

There are three big mission critical databases which includes Billing, Dynamics CRM, and an in house built application to keep track of transactions. These are high volume databases for a small/mid sized company. I need quantifiable or a good case study support in order to convince somebody that this is the wrong path to go towards. Any other advice on how I can convince this person would be helpful also.


The answer depends. On first glance, it may look like a bad idea. On the other hand, if the goal is to consolidate everything on one server, and then replicate that server in a remote environment, then you are on the way to a more reliable system. IT might prefer having everything in one place, rather than dealing with mission critical servers spread out over the terrain.

One major issue is the need for a larger machine. So, if any of the systems use software whose license depensd no the size of the machine, you nmight end up spending more money because you need a larger server. I've seen this happen with SAS licencing, for instance.

Perhaps the biggest issue, though, is that the different applications are probably on different development cycles -- whether developed in-house or from outside vendors. So, updating the hardware/operating system/software can become a nightmare. A fix or enhanced functionality in A might require an OS patch, which in turn, has not been tested on B. This maintenance issue is the reason why I would strongly advocate separate servers.

That said, mission critical applications are exactly that, mission critical. The driving factor should not be a few dollars on hardware. The driving factors should be reliability, maintenance, performance, sustainability, and recovery.

The comments made by Oded, Catcall and Gilbert are spot on.

The bank where I learnt the IT trade ran its entire core business on a single MVS (later Z/OS) mainframe, which ran a single DBMS and a single transaction processor (unless you counted TSO as a transaction processor).

The transaction processor went down regularly (say, once a day). It never caused the bank to go broke because it was always up and running again in less than a minute. Mileage may vary, but losing one minute of business time in an entire working day (480 minutes, or < 0.25%) really isn't dangerously disruptive.

The single DBMS went down too, at times (say, twice a month). I can still hear the sysprogs yelling "DBMS is down" over the fence to the helpdesk officers, meaning "expect to be getting user calls". It never caused the bank to go broke because it was always up and running again in a matter of minutes. Mileage may vary, but losing a couple of minutes of business time each month really shouldn't be dangerously disruptive.

The one time I do remember when the bank was really close to bankruptcy was when the development team had made a mess out of a new project in the bank's absolute core business, and the bank was as good as completely out of business (its REAL business) for three or four days in a row. That wasn't 0.25% loss of business time, but almost 100 TIMES more.

Moral of my story ? Two of them. (a) It's all about risk assessment (= probability assessment) and risk-weighted (= probability-weighted) cost estimation. (b) If you ask a question on SO (which implies a kind of recognition/expectation that answerers have more expertise than you on the subject matter), and people like Oded and Catcall provide you with a concise answer, which is accurate and to the point, then don't ask for papers or case studies to back up their answers. If you don't want to accept the experts' expertise, then why bother asking anything in the first place ?

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