4 Differences between the Azure VM Storage Types

Rob Waggoner

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If you haven’t heard, Microsoft now offers three types of storage for your Azure Virtual Machines and they include Standard HDD Storage, Standard SSD Storage, and Premium SSD Storage. Standard HDD Storage is based on the traditional hard disk model, Standard SSD and Premium SSD Storage are both based on Solid State Storage but offer different performance characteristics.

I’ve done some digging and want to break down the big differences for you. I found a great article that not only announces the Standard SSD Disks, but also breaks down all the details. I’m not going to repeat what’s in the article, but I will summarize a few of the details that impact your MyCloudIT deployments.

Difference #1
Standard SSD disks are only offered as Managed Disks. This is important because MyCloudIT uses Unmanaged disks when provisioning Standard HDD Storage. The reason we default to unmanaged Standard HDD Storage is that unmanaged Standard HDD Storage is charged by the amount of storage consumed. Managed Disks, and SSD storage are always charged based on the amount of storage allocated. For most deployments, more disk space is usually allocated than used; so, with Standard Storage, your Storage costs stay as low as possible. I’ve included a table that compares the various costs of managed vs. unmanaged disks for your review.

Storage PricingKeep in mind that all Managed disk is charged by the amount allocated, but unmanaged HDD storage is only charged for the amount of space consumed.

Difference #2
Let’s focus on the OS disk (C:) for each of your VMs. MyCloudIT provisions a 127 GiB C: drive for your VMs by default. The typical OS install consumes about 25 GiB of the 127 GiB allocation. I want to focus on the OS drive configuration because for all three storage types, Standard HDD, Standard SSD and Premium SSD; the 127GiB size is limited to 500 IOPS per disk. That means that no matter which VM series, and storage you provision, your disk IOPS will be the same. The only performance improvement Standard SSD or Premium SSD could provide to the OS disk is in relation to the latency of the IO transactions. Microsoft says that the latency will be less for the SSD-based disks but does not provide any metrics on the latency differences. Since the C: drive only contains the OS and maybe applications, I don’t feel most workloads will benefit from the reduced latency of the SSD based storage, especially when you factor in the additional cost of the storage. For Standard Storage (HDD), disk throughput is limited to 500 IOPS, the same disk throughput as Standard SSD and the same disk throughput of a 127 GiB Premium SSD. Now this is assuming that you are following best practices by not placing data on the C: drive. If you need a larger C: Drive to support data and data transactions, then you should consider a larger Premium Storage SSD drive for your C: drive, larger Premium Storage SSDs do provide higher IOPS.

Difference #3
If the IOPS of the OS disk is the same for all three storage sizes, why use SSD at all? If your workload, like SQL server or File Sharing services requires Premium Disk, your VM must be Premium Storage capable. Only Premium Storage capable VMs support attaching Premium Storage to them. Azure denotes Premium Storage capable VMs by putting an “s” in their name. For example, the D2s_v3 is a Premium Storage capable VM, while the D2_v3 is not capable of supporting Premium Storage Disks. Think about your RDSMGMT server, it supports a share that supports the User Profile Disks (UPDs). If you are supporting over 25 simultaneous users, Premium SSD is recommended because that many users will require more than 500 IOPS for a good user experience.

Difference #4
Some VM sizes, A-series VMs for example, can only use Standard HDD storage. While the A-series VM is economical, it has some definite limitations that need to be considered.

Hopefully the 4 differences between the Azure Storage Types I outlined can help you in deciding which storage type is right for you. Also, feel free to check out our User Guide for more How-to, Knowledge Articles, and Best Practices. 

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