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Scaling your deployment

The purpose of booting VMs, attaching disks and wiring up the network is to run jobs. Jobs on the virtual machines. Each job can do a certain amount of work based on the compute resources you've given it. If you need more work done, then you can add more virtual machines or increase the size/attributes of the virtual machines you are using. With BOSH, this is very easy.

NOTE: different jobs can run on the same virtual machine as each other. In the sample release tutorial we put the 3 jobs - nginx, wordpress and mysql on separate virtual machines.

Introduction to virtual machines on AWS

On AWS, there are a fix set of virtual machines you can use. These are called "instance types". In the example deployment manifest, wordpress-aws.yml, we set instance_type: m1.small for all three jobs. After we deployed the release, if you looked in your AWS console you would see the 3 additional running VMs.

There are several other AWS instance types. They are grouped together as families. For example, m1.small, m1.medium, m1.large and m1.xlarge are all in the "Normal" family. Each subsequent member of this family has 2x the CPU, RAM and I/O than its predecessor.

For example, the m1.small has 1.7G of RAM and 1 virtual CPU (VCPU). The next member of the family, the m1.medium, has 3.75G of RAM and 2 VCPUS. You want 4 VCPUs and 7.5G of RAM? Then you want the m1.large. AWS pricing also doubles with each increase in instance type. As of writing, m1.small is 8c/hr, m1.medium is 16c/hr, etc.

If your job requires a different ratio of CPU to RAM, then AWS has two other families of instance types - High CPU and High Memory.

The High CPU family currently has two instance types - the c1.medium and c1.xlarge. The c1.medium is like the m1.small with 1.7G of RAM, but has 5 VCPUs, and is only 2x the price of an m1.small at 16.5c/hr.

There currently no c1.large. Instead the c1.xlarge has 4x the RAM and CPUs of the c1.medium and is 4x the price.

The High Memory family currently has three instance types - the m2.xlarge, m2.2xlarge and m2.4xlarge. Each has 2x the CPU, RAM, I/O and price attributes of the former. The m2.xlarge offers 17.1G of RAM, only 6.5 VCPUs and costs 45c/hr.

If I/O is your job or entire environments primary bottleneck, then there is the Cluster Compute family of instance types.

In each family of instance types, the largest instance type has the highest I/O. To understand why, imagine how AWS is creating instance types. They take physical hardware - motherboards, CPUs, RAM and an ethernet cable - and virtualize it. The smaller the instance types, the more VMs that the hardware is supporting and the smaller the available I/O is to each VM. If you get the largest VM in a family, then you're getting all the available I/O (probably about 1Gbit) and are not sharing it with anyone.

Scaling with BOSH

Now that we know all about AWS instance types, let's change our deployment.

  • nginx: 1 x m1.small - that should be fine for our evented web server; as traffic scales it might want to grow to gain more I/O
  • wordpress: 1 x m1.small to 3 x m1.small - we can scale our wordpress jobs by adding more small instances
  • mysql: 1 x m1.small to 1 x m1.xlarge - give our SQL database a healthy combination of high I/O, high RAM and CPU.

That is, we're going to add 2 m1.smalls for wordpress, and upgrade the mysql job to a larger instance type.

This is easy with BOSH. We'll change our wordpress-aws.yml deployment manifest, re-run bosh deploy and the BOSH director orchestrates everything - from detaching and reattaching disk volumes (to keep our MySQL data safe) to deprovisioning and provisioning new VMs and their static IP addresses.

Make the following changes to your wordpress-aws.yml deployment manifest for the mysql job:

Add a new resource_pool for the special mysql instance:

resource_pools:
  - name: common
  ...
  - name: mysql
    network: default
    size: 1
    stemcell:
      name: bosh-stemcell
      version: 0.6.2
    cloud_properties:
      disk: 8192
      instance_type: m1.xlarge
      availability_zone:
      key_name: 

Change the mysql job to use this new resource_pool: mysql. Note that the explicit instance_type: m1.xlarge changes too.

jobs:
...
  - name: mysql
    template: mysql
    instances: 1
    resource_pool: mysql
    persistent_disk: 16384
    networks:
    - name: default
      default: [dns, gateway]
    cloud_properties:
      instance_type: m1.xlarge

Make the following changes for the wordpress job. We no longer need a m1.small for mysql, and need 1 for nginx and 3 for wordpress, which is 4 m1.smalls in total:

resource_pools:
  - name: common
    network: default
    size: 4

Next, allocate 3 instances (VMs) to wordpress.

jobs:
...
  - name: wordpress
    template: wordpress
    instances: 3

Run bosh deploy to confirm the differences and apply the changes:

$ bosh deploy

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