Azure Managed vs Unmanaged disks : The choice

Hi folks,

Recently (few months) , a new feature was announced to bring a new capability to Azure Virtual Machines : Azure Managed Disks.

Many blog posts explain well the purpose of managed disks, and how they bring enhancements to Azure IaaS virtual machines. I recommend the following readings :

The latter post shows the advantages of using Azure Managed disks, which I agree and confirm. But on the meanwhile, there is some ‘inflexible’ properties of managed disks, that may not be suitable for your or for your expectations. This is the purpose of this post : What is the model that fits my requirements , Managed or unmanaged disks.

1- The main difference between Managed disks and Storage Accounts based disks

There are some main differences between managed and unmanaged disks :

Category

Managed disks

Unmanaged disks

Management

Is an ARM (Azure Resource Manager) object (resource) Is not an ARM resource, but a file (.vhd) residing on a Azure Storage Account. The latter is an ARM  object

Size

The managed disks sizes are fixed (and can be resized). Which means that you cannot choose a custom size. You will need to pick up from a list. See (1) You can choose the disk size during the provisioning (and can be resized) when using Standard Storage. See (2)

Cost

You will pay :

·       Standard Storage :

o   A fixed price per disk size (Per month), whatever the disk usage is

o   Operations cost*

·       Premium Storage

o   A fixed price per disk size whatever the disk usage is

See (1)

You will pay :

·       Standard Storage :

o   The GB / month disk usage. You pay only what you consume

o   Operations cost*

·       Premium Storage

o   A fixed price per disk size whatever the disk usage is

See (3) and (4)

Performance

A managed disk have a predictable performance, with Standard storage (500 IOPS) or Premium storage (Depends on the disk). Only premium storage disks have a predictable performance (depends on the disk). Standard storage have a predictable performance (500 IOPS) unless they are impacted by the Storage Account performance limits (A maximum of 40 disks per standard storage account is recommended, otherwise disks can be throttled). See (5)

Availability

When placing VMs using managed disks under an Availability Set, disks are placed on different fault domains in order to achieve the better SLA (The Availability Set SLA is only for compute) When placing VMs using unmanaged disks under an Availability Set, there is no guarantee that the disks are placed on different fault domains, even if they are on different Storage Accounts.

Redundancy

LRS LRS, GRS

Encryption

ADE, SSE (Coming soon) ADE, SSE

* Operations cost means : Replication data transfer cost (In case of GRS) + Storage operations costs

 

2- Are managed disks more expensive that unmanaged disks ?

The answer is : It depends, but except in some cases, managed disks are always more expensive than unmanaged disks. Let’s prove it :

Standard Storage managed disk cost per month

  • Managed disk cost = Fixed Cost (Per disk size) + Operations cost
  • Unmanaged disk cost = Storage_Usage_In_GB * CostperGB + Operations cost

Because the Operations cost is the same for both models, we will omit them during calculation. Because the managed disk pricing model is not per usage, we will calculate the Disk size equity* value, to be able to compare with unmanaged disk :

Managed disk type

Size (GB) Price Cost per GB Standard Storage price per GB Disk size equity*

S4

32 1.3 0.040625 0.0422 31

S6

64

2.54

0.0396875

60

S10

128

4.98

0.03890625

118

S20

512

18.36

0.035859375

435

S30 1024 34.56

0.03375

818

* If you use less than the given size, then Unmanaged disks will cost less than managed disks. If you use more, then managed disks cost will be less than unmanaged disks. The €/GB will be greater as long you consume less storage space.

NB : New disk sizes have been announced (6) that finally make and end for the 1 TB disk size limit, with 2 and 4 TB for managed disks, and up to 4 TB for unmanaged disks. The service started on the West US Central region and will be generalized for the remaining regions during the coming months

3- Do I really need managed disks ?

This is a good question, but the answer is very relative to your needs. As you probably have read on the posts I mentioned earlier in this post, there are many benefits of using managed disks:

  • Disk snapshots
  • Predictable performance
  • Distribution in different fault domains when associated with Availability Sets
  • ARM object

Some workarounds may be used to have similar properties with unmanaged disks:

Properties

Unmanaged disk workaround

Disk snapshots

 

No workarounds

Predictable performance

 

Place less than 40 disks per Storage Account

Distribution in different fault domains when associated with Availability Sets

 

No Workaround. There is no way to know if the disks are place on different pools even if they are on different Storage Accounts

ARM object

 

Place each disk on its own Storage Account. Look if this will fit your needs  (Do not forget quotas)

 

4- Verdict

As you can see, managed disks brought new experience and features to Azure VM storage that permits better controlling the VM storage. Personally, I would recommend using managed disks, even if the ‘Pay as you consume’ model is not adopted there. But the features and the simplicity is worth the ‘little’ difference we can see with pricing.  Continue reading

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