Azure premium storage is very performant and is Azure's alternative to slower standard storage backed VHDs. However, there are some things you should know about how Azure premium storage is priced which is considerably different from other cloud offerings such as Amazon Elastic Block Store (EBS).

Azure Data Disks

When you select an 'SSD' disk in the portal what you're actually doing is selecting a VHD image that is backed by Azure premium storage.

Azure VM Disk Type Selection Dialog

Unfortunately on Azure, OS Disks are limited to 30 GiB. But what is surprising is that this disk, and this disk alone will cost you $20/month.

Price Inflexibility

This has to do with the price inflexibility of Azure Storage. Premium storage pricing falls into 3 buckets:

  • P10 - 0-128 GiB ~ $20/month
  • P20 - 129-512 GiB ~ $75/month
  • P30 - 513-1028 GiB ~ $135/month

(These prices vary depending on your data center of choice)

Now, when you go into the Azure portal, and spin up that DS series VM with a 30GiB 'SSD' OS Disk -- that's an extra $20 bucks, just for 30GiB of storage (albeit -- fast storage).

I/Ops Turbo Boost

However the IOps that you get on Azure Storage is quite impressive. The smaller the disk, the higher the IOps you'll get. Although a 30GiB data disk will run your team an additional $20/month you'll likely get around 5000 I/O operations per second (I/Ops) performance. Perfect for databases, CI servers and such. This is compared to 500 I/Ops for a similar sized standard storage disk (which costs around $2-$3/month).

Guidance Around Premium Storage VMs

Premium storage costs can add up fast. Some VMPower users are spending over $800 per month in unused premium storage disks.

Unless you really need high I/Ops on your OS disk (you probably don't) it is a smart idea to provision your VMs with standard storage on the OS disk and use Premium storage for data disks.

This way you don't end up paying such a high cost per gigabyte of high-performance storage.