Concurrency and Storage Needs

Concurrency and Storage Needs :

We operate on K-12 Online Summative Market.
Our Needs are:
When Kids log-in to take the test, they should be able to login without any issues.
Should be able to store their responses and score them on the fly without much latency.
Should be able handle entire state. If a state decides to test their 8th Graders with High Schoolers on one Morning, System should be capable of handling entire state 8th graders and High Schoolers .
Concurrency Needs vary but 500 K Concurrent Users is good number for us to support.

In simple Terms, COST , SCALE , SPEED and RELIABILITY are our considerations not necessarily in that order.

It should be fast.
It should be reasonably priced.
Data should be safe and Secure. Can’t afford to lose any data at all.

How can we achieve these needs.
Options considered:
Direct Attached Storage
Fusion IO

Good Primer on Scale Out vs Scale Up:

Microsoft Article on Scaling Out SQL Server:

Stack Exchange Decision Making Process on their Storage System. Their needs are pretty similar to our needs.

Brent Article on Scaling SQL Server :

Brent Ozar Fusion IO Benchmarking Tool: