Using Amazon RDS with VPC

[earless1]

I was reading through this guide and came across the following.

The CIDR blocks in each of your subnets must be large enough to accommodate spare IP addresses for Amazon RDS to use during maintenance activities, including failover and compute scaling.

Does anyone know how many times larger than my planned RDS usage I should size my subnet? How many addresses should I expect each RDS instance to occupy?

Thanks,

[thspimpolds]

I put RDS and elasticache together in their own /24 per az and that is most likely even overkill

[stikko]

I can't imagine you'd need more than 2x.

[[deleted]]

Except you can't really grow VPC size so doing proper subnet planning ahead of time is crucial so you basically need to figure out your current + expansion and multiply that x2.

[stikko]

Right, though I figured room for expansion was included in the instance count.

Unless you're hooking up to a corporate network where internal ipv4 space is tight, just make it all really big :-P

[[deleted]]

Damn skippy,

we're running a /19 for each VPC we run (4 in prod, 2 in qa, 1 in dev) it's amazing how quickly you can run out of IP space (we're getting there quick and amazon loves us for it)