Rackspace cost comparison with Amazon EC2

(Earlier posts in this series are: EC2 cost break down, GoGrid & EC2 cost comparison)

We looked at Amazon EC2 and GoGrid cost earlier. Let us examine another IaaS provider — Rackspace cloud. The first step again is to unify on the same unit of measurement on the CPU power. Using the same methodology as we used for EC2’s hardware analysis, we determine that Rackspace runs on a platform with two sockets of Quad-Core AMD Opteron 2374 HE processor. According to PassMark-CPU Mark results, this platform has a CPU mark score of 4642, which is roughly 12 ECU. Rackspace cloud’s FAQ states that “For Linux distributions, each Cloud Server is assigned four virtual cores and the amount of CPU cycles allocated to these cores is weighted based on the size of the Cloud Server.” From talking to Rackspace support, we know that each physical host has 32GB of RAM, and it can host at most 2 16GB (15.5GB to be precise) VMs. Therefore, a 16GB VM would own the complete 4 cores it is allocated, i.e., the 16GB VM has a guaranteed capacity of half of the platform, which is 6 ECU. Since Rackspace states that the CPU is proportionally shared based on the RAM, we can derive the minimum guaranteed CPU based on how many other VMs could fit on the same physical host. The following table lists the minimum CPU and the maximum CPU (assuming full bursting when all other VMs are idle). Again, we are only concerned about Linux VMs, as they do not include license costs, so they more accurately represent the true hardware cost.

RAM (GB)	Storage (GB)	Min CPU (ECU)	Max CPU (ECU)	Cost (cents/hour)
0.256	10	0.09375	6	1.5
0.512	20	0.1875	6	3
1	40	0.375	6	6
2	80	0.75	6	12
4	160	1.5	6	24
8	320	3	6	48
16	620	6	6	96

Similar to GoGrid, Rackspace only charges based on the RAM, so it is not possible to determine how it values each component (i.e., CPU, RAM and storage) separately, as we have done for EC2. However, it is possible to project what a similar configuration would cost in EC2 using the unit cost we have derived from the EC2 cost breakdown. The results are shown in the following table where we assume a VM only gets its minimum guaranteed CPU. Each row corresponds to one VM configuration, which is denoted by its RAM size in the first column. We also show the ratio between the Rackspace cost and the projected equivalent EC2 cost.

RAM (GB)	Rackspace cost (cents/hour)	Equivalent EC2 cost (cents/hour)	Rackspace cost/EC2 cost
0.256	1.5	0.8	1.87
0.512	3	1.6	1.87
1	6	3.16	1.9
2	12	6.32	1.9
4	24	12.6	1.9
8	48	25.3	1.9
16	96	50.2	1.91

Since a Rackspace VM can burst if other VMs on the same host are idle, it could potentially grab a much larger share of the CPU. The following table shows the cost comparison assuming that the VM bursts to its fullest extent.

RAM (GB)	Rackspace cost (cents/hour)	Equivalent EC2 cost (cents/hour)	Rackspace cost/EC2 cost
0.256	1.5	8.89	0.17
0.512	3	9.56	0.31
1	6	10.86	0.55
2	12	13.5	0.89
4	24	18.8	1.28
8	48	29.4	1.63
16	96	50.2	1.91

If your VM is only getting the minimum guaranteed CPU, Rackspace is about 1.9 times more expensive than an equivalent in EC2. However, in our experience, we can frequently grab a much larger share of the CPU. Assuming you can grab the full 4 cores, the 256MB, 512MB, 1GB, and 2GB VMs are a great bargain, which are 17%, 31%, 55%, and 89% of the equivalent EC2 cost respectively.