{"id":10802,"date":"2010-08-23T10:21:48","date_gmt":"2010-08-23T10:21:48","guid":{"rendered":"https:\/\/aidanfinn.com\/?p=10802"},"modified":"2010-08-23T10:21:48","modified_gmt":"2010-08-23T10:21:48","slug":"hyper-v-cluster-with-different-capacity-hosts","status":"publish","type":"post","link":"https:\/\/aidanfinn.com\/?p=10802","title":{"rendered":"Hyper-V Cluster with Different Capacity Hosts"},"content":{"rendered":"

Last week I was asked about how you would introduce new, bigger, Hyper-V hosts to a cluster.  For example, there was a time (not long ago) when the sweet spot for RAM in a host was 32GB RAM.  You might have a number of these hosts in a cluster.  For example, a cluster with 8 or less hosts would have 1 redundant node with 32GB RAM.  If one host fails, then the redundant host can take up the slack.<\/p>\n

In reality, virtual machines will be running across all of the hosts in a load balanced environment.  You will have the equivalent of 1 host in redundant capacity<\/em>.<\/p>\n

A cluster with 9-16 nodes will probably have 2 redundant hosts (or equivalent capacity).<\/p>\n

Say I have 5 hosts with 32GB RAM with 1 of those being redundant.  Now I can purchase hosts with 64GB of RAM at a decent rate because servers have many more memory slots and I don\u2019t need to buy the exponentially more expensive 8GB or 16GB memory boards.  Can I buy just one of those servers and add it to the existing cluster of 32GB RAM hosts?  Sure you can.  But you will have trouble when you add more VM\u2019s to it than you could add to a 32GB RAM host.<\/p>\n

Let\u2019s put it this way:  Say I have 2 * 1 gallon buckets.  1 is full of water.  I can put .5 gallon in each bucket.  I can pour .5 gallon from one bucket to another to wash it or repair the original bucket.  I always have 1 gallon of water \u2026 clustered between my 2 * 1 gallon buckets.  Now I want to carry much more water and I buy a 2 gallon bucket to add to my collection of buckets.  I have 1 * 1 gallon bucket that is full.  I have 1 * 2 gallon bucket that is full.  I have 1 empty 1 gallon bucket as a spare.  But it can only be a spare for the other 1 gallon bucket.  I will have to throw away 1 gallon if I need to wash or repair the 2 gallon bucket and pour its contents into the spare 1 gallon bucket.<\/p>\n

The same goes for Hyper-V hosts (purely on RAM capacity).  A 32GB RAM host cannot offer full redundancy for a 64GB RAM host.  Half of the virtual machines can migrate and stay running\/reboot, but the other half will not be able to start.<\/p>\n

Here\u2019s what you can do in a growth scenario:<\/p>\n