Exactly the right question
Yes, this is exactly the right question to ask, for two reasons:
1.) No, Flash is not cheaper than (SATA) disk, at least when you compare $/GB. No matter how hard the AFA vendors try to tell you otherwise, a simple price comparison on Amazon will tell you that this is not true. Yes, Flash prices are going down, but so are disk prices. Look at any chart that shows disk and flash price over time and you'll see that the price gap between the two hasn't fundamentally changed - it likely won't any time soon.
So if you AFA appears to be cheaper than a traditional array on a $/GB basis it's either due to some creative accounting on the AFA vendor side or because the traditional array is seriously overpriced.
2.) For typical enterprise applications, the storage part of the total latency is not very relevant. Look at a typical ERP system and you'll probably find a latency on the client side of around 80ms. Storage latency on a disk/hybrid system will contribute around 6ms of that. The rest is network, server processing etc. Getting the storage latency from 6ms to 1ms is not really helping you a lot if your total client-side latency is 80ms. Better invest that money to optimize in other areas.
Disk based arrays (hybrid with Flash as a cache) will not go away anytime soon - just like tape never went away. The ability to move workloads between different storage tiers will be key as most customers will need different tiers. If your array only supports a single tier (--> AFA startups) it won't be a full solution to your storage needs. It seems to be relatively easy to just add flash as a new tier to a slightly tuned traditional array (and achieve very similar performance to startup AFAs) but it will likely be very hard to add disk to a pure AFA.
Biting the hand that feeds IT
