This is a very frequent question that I get in my job and I thought I would provide some clarity on this on my blog.

Azure Premium Storage performance is determined by two factors:

• Disk Size
• VM Size

Choosing the right disk size

Currently (April 2016) there are three premium storage disk sizes. If you choose to create a disk with a different size, your disk will be charged for and support the performance targets of the next tier up. This is due to the fact that Azure needs to allocate a whole SSD of the next size tier to host your disk image.

Premium Storage Disk Type	P10	P20	P30
Disk size	128 GiB	512 GiB	1024 GiB (1 TB)
Max IOPS per disk	500	2300	5000
Max throughput per disk	100 MB per second	150 MB per second	200 MB per second
Max number of disks per storage account	280	140	35

Source: https://azure.microsoft.com/en-us/documentation/articles/storage-scalability-targets/#scalability-targets-for-azure-resource-manager

Choosing the right VM size

Depending on what premium storage VM you choose in conjunction with your premium disks, you may impose a limit on the performance that you can achieve.

For example: A DS1_v2 instance is limited to 48 MB per second throughput and 3,200 IOPS. This means that even with a P30 disk attached to it, it will never achieve 200 MB per second or 5,000 IOPS.

Source and full table for all premium storage machine sizes: https://azure.microsoft.com/en-us/documentation/articles/virtual-machines-windows-sizes/#standard-tier-ds-series