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HFI - Cloud Computing (CC)

Showback

Assignment

"The board wants to know what every member uses," Beat Kläy writes. "We don't charge anything yet, but every member should see its costs."

Learning goals

After this chapter you can:

  • Tell showback and chargeback apart
  • Store prices per resource in a template
  • Calculate and read the costs of the members with oneshowback
  • Work out the cost of a VM per hour and per month yourself
  • Check with a simple cost model whether the prices cover the costs

Showback and chargeback

Section titled “Showback and chargeback”

Anyone who shares a cloud wants to know who uses how much of it. There are two approaches. Showback shows every business unit what it has used, without any money changing hands. Chargeback charges this usage to the unit's budget. Showback comes first, because only when the numbers are right and everybody understands them can you base an invoice on them. Whether an organization introduces chargeback afterwards depends on its accounting.

Wolkenwerk Bern starts with showback. The members see their costs every month, and the board decides later how the cooperative charges them.

Prices in the template

Section titled “Prices in the template”

OpenNebula works with three hourly prices stored in the template. CPU_COST applies per CPU of the template, MEMORY_COST per MB of memory and DISK_COST per MB of disk. The prices in the lab are made up for the case and are meant in Swiss francs. The showback documentation describes how to set the prices of a template.

costs.tpl
# Prices for showback per hour, chapter 3.3. The amounts are invented for the case.
#
#   onetemplate update ubuntu --append ~/cc/lab/tenants/costs.tpl


# per CPU of the template and hour
CPU_COST    = "0.05"
# per MB of memory and hour
MEMORY_COST = "0.00002"
# per MB of disk and hour
DISK_COST   = "0.000001"
Terminal window
onetemplate update ubuntu --append ~/cc/lab/tenants/costs.tpl
onetemplate show ubuntu | grep COST

A VM takes the prices from the template when it starts. VMs that were started earlier therefore cost nothing, and a reboot with onevm reboot does not change that. In the lab you terminate the members' VMs with onevm terminate after adding the prices and start them again from the template.

Calculating costs

Section titled “Calculating costs”

oneshowback calculate calculates the costs of all VMs from their running time and stores them per month. OpenNebula ships the timer opennebula-showback.timer, which runs this calculation every day. For the exercises you start it yourself so that you do not have to wait until midnight. oneshowback list then shows owner, group, month, hours, running time and cost per VM. With -g you only see one member. The guide to the monthly reports shows how to calculate and retrieve single months.

Terminal window
oneshowback calculate
oneshowback list
oneshowback list -g treuhand
oneshowback list --csv > ~/cc-lab/showback.csv

With oneshowback list a group admin only sees the VMs of their own group. Each member can therefore check its own costs without seeing those of the others.

Sunstone shows the costs in the "Showback" tab of a group.

The Showback tab of the group praxis in Sunstone. The top shows the costs per month, the bottom the costs and hours of each VM. The lab had only run for about an hour with prices, so the amounts are small. The VM praxis-1 cost 0.05 francs in that hour, which matches the calculation in the next section.

The cost of a VM

Section titled “The cost of a VM”

A VM from the template ubuntu has 0.5 CPU, 1024 MB of memory and a disk of 3584 MB. Per hour it costs

0.5 × 0.05 + 1024 × 0.00002 + 3584 × 0.000001 = 0.025 + 0.0205 + 0.0036 ≈ 0.049

Over 30 days, or 720 hours, that comes to about 35 francs. The cost follows the time a VM holds resources on a host. A VM switched off with onevm poweroff therefore keeps costing. Only onevm undeploy takes it off the host, and then it costs nothing in this model. It pays for the members to take VMs they do not need for a while off the host that way.

The showback calculation At the top left the accounting of OpenNebula supplies the running time of the VM treuhand-1, 720 hours in one month. Next to it are the hourly prices from the template ubuntu. oneshowback calculate turns these into CPU hours, MB hours of memory and MB hours of disk and multiplies them by CPU_COST, MEMORY_COST and DISK_COST. That gives 18.00, 14.75 and 2.58 francs, together 35.33 francs for the VM in one month. At the bottom right, oneshowback list -g treuhand lists the cost of every VM of the group, and together they make up the member's showback. A group admin only sees the VMs of their own group. Notice that every row uses the same 720 hours, the time in which the VM holds resources on a host. ACCOUNTING per VM PRICES per hour oneshowback calculate treuhand-1 running time 720 h Template ubuntu from costs.tpl Resource Amount × hours Price Francs CPU 0.5 × 720 = 360 × 0.05 18.00 Memory 1024 × 720 = 737280 × 0.00002 14.75 Disk 3584 × 720 = 2580480 × 0.000001 2.58 treuhand-1 in a month 35.33 SHOWBACK oneshowback list -g treuhand A group admin only sees the VMs of their own group. Group treuhand sum of its VMs

In the end the prices have to cover the cooperative's costs. These include the hardware over its lifetime, power, licenses, backup and the work of the platform team, as well as the reserve for maintenance from chapter 2.5. A simple approach divides the yearly costs by the capacity that can be sold in a year. Chapter 6.4 works with all the costs, and advanced task C of milestone 4 does the same for a member of your own. In advanced task B you work out from a monthly report how far the prices cover the cooperative's costs.

Exercise Lab

Add the prices to the template. As praxis-admin, terminate the VM praxis-1 with onevm terminate and start it again from the template under the same name. Also start the VM treuhand-1 as treuhand-admin. Calculate the showback after one hour, and carry on with chapter 3.4 in the meantime. Export the list as CSV to ~/cc-lab/showback.csv and capture it in a screenshot for milestone 1.

Solution

onetemplate show ubuntu shows CPU_COST, MEMORY_COST and DISK_COST. After oneshowback calculate, oneshowback list lists the new VMs with about one hour and costs around 0.05. The old VMs without prices appear with cost 0.

Exercise Cost model

The two nodes cost 7,000 francs together and run for five years. Power, backup and work cost 6,000 francs a year. On average 10 VMs from the template ubuntu are in use. Do the lab prices cover the costs? What would you propose to the board?

Solution

The costs per year are 7,000 / 5 + 6,000 = 7,400 francs. 10 VMs at 35 francs a month bring in 4,200 francs a year. So the prices cover only a little more than half. Either the prices go up to about 1.8 times their current level, or utilization has to grow. A fixed base fee per member for the work of the platform team would be another option.

Text and figures are licensed under CC BY-SA 4.0 and the code under the MIT License. The sources are on GitHub.