While advances in technology are seemingly continuous, and the limitations of yesterday’s systems are now the minimum requirements of tomorrow’s, care should still be taken to make sure every capability scales as much as you need it to.
Virtualisation of yesterday
In the last few years, the capabilities of virtualisation have increased dramatically. It was only a few years ago that virtual servers were limited to 4 vCPUs, had only average performance storage, and never had enough memory. That explains why for a long time it was only ever our “small footprint” servers that we virtualised.
Virtualisation in 2013
Today, the enterprise virtualisation platform vendors deliver capabilities that we only dreamt of a few years ago; 32 vCPUs, 1TB of memory, and high availability by default, for every virtual server.
That’s a lot of engine power we can put behind our virtual servers, allowing us to virtualise hopefully most of our data centres if we choose to these days.
Watch for the subtle limitations
However, despite these great headline capabilities, there are still inherent capability limits that the small print carries. I found one of these during a cluster build recently and that prompted me to write this article.
Our project was using VMware 5 to host some fairly large nodes of a SQL Server failover cluster instance. Each virtual server had 16 vCPUs and 128GB of memory, something that VMware can adequately provide, but it was the storage configuration where we hit a limitation by surprise.
The SQL Server cluster design we initially created required upwards of 60 mount points to be presented to each of the cluster nodes, however we then discovered that VMware has a limit of 45 clustered/shared SAN volumes (raw device mappings) being presented to a virtual server.
What it quickly demonstrates is that while our focus was initially on whether or not the virtual server would run fast enough, the limitation that actually mattered to us was actually related to storage provisioning.
Published configuration limitations
The links below provide details from VMware and Microsoft about the complete limitations of their virtualisation software, and are worth reading if you’re planning to deploy virtual servers with more than the default resources.