Azure Managed Disks
Azure Managed Disks – Features and Cost

On 11th February 2017 Microsoft announced the General Availability of a new Azure Service: Managed Disks.

This is an alternative offering to storing Virtual Machine Hard Drives in Storage Accounts.

Advantages

Simplified management

You no longer need to create and manage individual Storage Accounts. This allows you to circumvent the 20,000 IOPs per Storage Account limit as well as the default 100 Storage Accounts per subscription limit.

Scalable VM deployments

It is now possible to create much larger Virtual Machine Scale Sets with up to 1,000 Virtual Machines.

Granular Security

You can now use Role Based Access Control as a per Disk setting whereas before you could only use it for Storage Accounts.

Simplified Standard to Premium Storage Migration

It is now possible to switch the type of VHDs between Premium and Standard Disks. This used to require copying the blob between Storage Accounts, changing the VHD Uri etc. but now only requires a shutdown. This makes it a lot easier to use Premium Storage when actively utilizing a VM and switching to Standard Storage when the VM will not be used for a while.

Simplified Standard Disk Resizing

Resizing a Standard Disk can now be done using the Portal by choosing a different size as opposed to PowerShell.

New Feature: Snapshot

You can now create Snapshots of running Virtual Machines!!!

Creating a snapshot has always been easier for Classic VMs via the Portal but required a lot of additional configuration and PowerShell for Resource Manager VMs.

You can now Capture both running and stopped Resource Manager VMs from the Portal.

The Costs for keeping the Snapshot are the same as for normal Managed Disks i.e. if you are making a snapshot of a 128GB Managed Disk you will be billed as though you had created a second one. You can however make a Standard Disk Snapshot from a Premium Disk and vice versa.

These costs can be reduced further by copying the resulting BLOB to a Standard Storage Account and subsequently deleting the Snapshot.

Pricing

This is of course one of the most interesting points for many of us.

For Premium Storage there is no difference, which is why I would recommend using Managed Disks right away in this case. You can migrate existing VMs using PowerShell, be aware however that this will not delete the original Disks or Storage Accounts.

Migrating a VM will not delete original Disks or Storage Accounts. Be aware of the costs this might incur!

For Standard Storage there are now different prices based on the size of the disk.
The following prices are for West Europe.

S4 S6 S10 S20 S30
Disk Size 32 GB 64 GB 128 GB 512 GB 1,024 GB
Price per month* €0.648 €1.269 €2.483 €9.176 €17.28
* Promotional Price: 50% until July 2017, West Europe

Comparison

Note that currently promotional pricing applies. I will be comparing the long term pricing for further discussion.

GB Utilized Managed Disk Unmanaged Disk Difference
16 € 1.30 € 0.67 € 0.62
32 € 1.30 € 1.35 – € 0.05
48 € 2.54 € 2.02 € 0.52
64 € 2.54 € 2.70 – € 0.16
96 € 4.96 € 4.05 € 0.91
128 € 4.96 € 5.40 – € 0.44
192 € 18.35 € 8.10 € 10.26
256 € 18.35 € 10.80 € 7.56
320 € 18.35 € 13.49 € 4.86
384 € 18.35 € 16.19 € 2.16
448 € 18.35 € 18.89 – € 0.54
512 € 18.35 € 21.59 – € 3.24

The gist of it is that if you are within 20% of utilizing the full capacity you are saving money by using Managed Disks.

Caveats

Managed Disks only support Locally Redundant Storage for the foreseeable future.

I would never use a OS Disk that has less than 128 GB capacity because of the catastrophic events that can and will ensue when an OS disk is full. With an OS Disk I usually don’t utilize more than 32 GB though.

This means that I am paying almost 4 times as much (€4.96 vs. €1.35) for allowing myself some breathing space and thereby underutilizing the Resource.

When I use Standard Disks for Data I usually employ the maximum number supported by the given VM Size and then apply striping to get the maximum performance.

Assuming I utilize about 16GB of Data and the VM supports 8 Managed Disks, if I use 8 * S4(32GB) managed disks I would be spending at least €10.40 vs. €0.67 with unmanaged disks.

Conclusion

For now, I will be using Managed Disks for Premium Storage but sticking to Unmanaged Disks for Standard Storage because of the cost effectiveness.

References

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Alexander Jebens

Alexander Boersch is a freelance consultant, software architect and engineer based in Vienna, Austria. He facilitates and drives cloud transition and adoption within organisations while ensuring performance, consistency, governance and cost control. He supports development projects in choosing their stack, securing their solution and adopting DevOps. When not conducting workshops or hacking away at his keyboard, he enjoys running and ski touring while he ponders upon the mysteries of life, the universe and everything.

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