In my experience with Accumulo on EC2, I've seen about an 85% increase in aggregate write rate each time the size of the cluster is doubled. I've tried to capture that behavior in a model to help myself understand it.
The model I came up with is the following:
w: aggregate write rate (writes per second)
m: number of machines
k: standalone single server performance (in my experience about 30k writes per second on average)
the units of k and w are writes per second
for those of you without the ability to see graphics in email, the model is:
w = m * pow(0.85, log(m, 2)) * k
First of all, my algebra may be rusty, so it may be possible to simplify the model ... second, does the model make sense?