How do you measure availability?

Recently on Twitter, there was a conversation about vendors and certifying the availability of their arrays; which vendors certify their arrays as five nines etc. I am going to argue that these figures lull people into a false sense of security as actually no-one knows what they mean!

If a vendors says that that their array is 99.999% available, what does that really mean to you? Probably not a lot in practical terms. Does it mean that individual components are 99.999% available? Or does it mean that the array itself in some shape or form is available?

If the array is still powered on and not in flames, is that available?

If 75% of disks are working, is the array still available?

If the array can service any I/O is that available?

What do vendor figures actually mean and do they matter to you? More importantly, do they matter to your customers? Your customer doesn’t care whether the array is still working, all they care about is whether they have access to their data and their service is available. So ultimately, vendor availability figures are pretty much meaningless in the larger scheme of things.

So those vendors who read my blog, what do your availability figures actually mean?

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Martin Glassborow

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