Tiered Storage/Broadband Model

Comments on my last post reminded me of a conversation I had recently regarding the setting of specific performance criteria for storage tiers. Some companies attempt to set specific performance guarantees; for example setting tier 1 storage to a response time of 10ms or less, tier 2 to 20ms or less.

I think there are two problems with this approach. Firstly, within a shared infrastructure it is *impossible* to guarantee that all I/O operations will complete within a pre-determined performance range (like the 10ms example). It may be possible to say 99.9% of I/Os measured over a 5 minute period are guaranteed, but there will always be the odd spike which will make 100% an unrealistic target.

Secondly, I think most users have no idea what a response time of 10ms or 20ms has on their application when they are planning a new project and consequently they will take the best tier possible – unless there is a big difference in the cost.

Take the following analogy. Imagine you are looking for a new broadband Internet service. Your ISPs now offer from 512Kb/s up to 24Mb/s. Which service do you take? I think most people will take the fastest service possible – until they see the cost. Imagine the price structure was as follows;

• 24Mb/s – £100
• 8Mb/s – £20
• 2Mb/s – £10
• 512Kb/s – £2

In this scenario, I’d be happy with the 8Mb/s service, because in reality I have no idea whether I *need* 24Mb/s (but rather I just *want* it).

The same logic applies for tiering. Most users think they will need tier 1 and unless they have a price/cost disincentive to not use it, then tiering won’t work.

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