CloudPatterns.org

Problem

Cloud consumers are commonly charged for cloud-based storage space based on fixed-disk storage allocation. This means that they are charged based on the capacity of the fixed disks, regardless of their actual data storage consumption.

For example, a cloud consumer may provision a virtual server with the Windows Server operating system and three 150 GB hard drives. When the cloud consumer installs the operating system, it is billed for using 450 GBs of storage space, even though the operating system may only use 15 GBs.

Figure 1 - A scenario based on the use of a fixed-disk allocation based provisioning model.

1. The cloud consumer requests a virtual server with three hard disks, each with a capacity of 150 GB.
2. The virtual server is provisioned via the Rapid Provisioning and Automated Administration patterns, with a total of 450 GB disk space.
3. The 450 GB of storage space is allocated to the virtual server by the cloud provider.
4. The cloud consumer has not installed any software yet, meaning the actual used space is currently 0 GB.
5. Because 450 GB has been allocated and reserved for the cloud consumer by the cloud provider, the cloud consumer will be charged for 450 GB of disk usage as of the point of allocation.

Solution

A system of dynamic storage provisioning is established so that the cloud consumer is billed, on a granular level, for the exact amount of storage that was actually used at any given time. This system uses thin provisioning technology for the dynamic allocation of storage space and is further supported by runtime usage monitoring to collect accurate usage data for billing purposes.

Figure 2 - A scenario based on the use of a dynamic-disk allocation based provisioning model.

1. The cloud consumer requests a virtual server with three hard disks, each with a capacity of 150 GB.
2. The virtual server is provisioned via the Rapid Provisioning and Automated Administration patterns, with a total of 450 GB disk space.
3. 450 GB of disk space is set as the maximum allowed disk usage for this virtual server, but no physical disk space has actually yet been reserved or allocated yet.
4. The cloud consumer has not installed any software yet, meaning the actual used space is currently 0 GB.
5. Because the allocated disk space is equal to the actual used space (which is currently at zero), the cloud consumer is not charged for any disk space usage.

The cloud consumer will only be charged when the disk space has actually been used.

Figure 3 - The fixed-disk allocation based provisioning model compared to the dynamic disk provisioning model.

Application

Thin provisioning software is installed on servers that need to process dynamic storage allocation. A specialized cloud usage monitor is employed to track and report actual disk usage.

Figure 4 - A sample cloud architecture resulting from the application of the Elastic Disk Provisioning pattern.

1. A request is received from the cloud consumer and the provisioning for a new virtual server instance begins.
2. As part of the provisioning process, the hard disks are chosen as dynamic or thin provisioned disks.
3. The hypervisor calls a dynamic disk allocation component to create thin disks for the virtual server.
4. Virtual server disks are created via the thin provisioning program and saved in a folder of near-zero size. The size of this folder and its files grows as operating applications are installed and additional files are copied onto the virtual server.
5. The pay-per-use monitor mechanism tracks the actual dynamically allocated storage for billing purposes.

Because the allocated space is equal to the amount of used space after applying the pattern, the cloud consumer will only be charged for 15 GB of storage usage. Should the cloud consumer later install an application that takes up 6 GB of disk space, it will be billed for using 21 gigabytes from thereon.

NIST Reference Architecture Mapping

This pattern relates to the highlighted parts of the NIST reference architecture, as follows: