I wanted to give a boost to one of my Azure virtual machines that is being used as a build server for processing thousands of small files in bursts. So I decided to add a small 32 GiB premium storage (SSD) disk.

But while reading Microsoft documentation for Azure disks, something puzzled me:

For a 32 GiB standard disk (S4):

IOPS per disk 500

Throughput per disk 60 MB/sec

For a 32 GiB premium disk (P4):

IOPS per disk 120

Throughput per disk 25 MB/sec

Does this mean that for small disks there is no point of upgrading to premium SSD because performance will be actually lower? It does not make logical sense to me. Is it true or I misunderstand something?

  • From what I've seen, you can achieve up to 80K iOPS per VM with premium storage. So it seems that the performance increases exponentially in this case. Mar 8 '18 at 18:13

Yes, the performance of standard storage is the same per disk across the board, whereas premium storage increases as the disk size increases. This does mean that for smaller premium disks the performance may be low than the same size standard disk.

The main reason you would use premium disk at this size is for the SLA. To be able to get the 99.9% SLA for single instance VM's then you must use premium disks. Generally premium storage is recommend for production work loads.

  • Currently, the SLA for Managed Standard SSD is only .4% less at 99.5% instead of the 99.9% (azure.microsoft.com/nl-nl/support/legal/sla/virtual-machines/…). However, the standard disks also don't burst (azure.microsoft.com/nl-nl/pricing/details/managed-disks) which makes them a lot less attractive for most real world workloads especially combined with the transaction costs (for read/write/update). Jun 15 at 12:53
  • I’m not taking about the disk SLA, if you want to get an SLA on a single instance VM you must use premium storage. You cannot use standard SSDs
    – Sam Cogan
    Jun 15 at 12:55
  • Looking at the linked SLA, that does not currently seem to be the case: "For any Single Instance Virtual Machine using Standard SSD Managed Disks for Operating System Disk and Data Disks, we guarantee you will have Virtual Machine Connectivity of at least 99.5%." Jun 15 at 12:57
  • Yep you are correct, looks like that got updated.
    – Sam Cogan
    Jun 15 at 12:58

Just to add to Sam Cogan's response, the SLA for single-instance VMs is covered here. Furthermore, Standard storage disks are subject to transaction costs, which Premium disks are not. That is covered on the pricing page.

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