This chapter covers Azure Managed Disks, a core storage service that provides persistent, high-performance block storage for Azure Virtual Machines. Understanding managed disks is critical for the AZ-900 exam, as it falls under Objective 2.4 (Describe Azure storage services) and appears in approximately 10-15% of questions related to compute and storage. By the end of this chapter, you will know how managed disks work, their types, how they differ from unmanaged disks, and how to choose the right disk SKU for your workload—all essential for both the exam and real-world Azure deployments.
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Imagine you are a business traveler who needs a place to stay for a year. You have two options: rent a hotel room or lease an apartment. With a hotel room, the hotel provides everything—bed, desk, TV—but you share the building's infrastructure (elevators, hallways) with other guests. If you want to upgrade your room, you must move to a different room entirely. This is like an unmanaged disk: the VM (your room) is tightly coupled with the storage (the furniture). If you need more storage, you must recreate the VM. Now consider leasing an apartment. You sign a lease for a specific unit, but you can bring your own furniture, rearrange it, or even renovate the kitchen. The building's infrastructure is separate from your unit. If you move to another apartment in the same building, you can take your furniture with you. This is a managed disk: the storage (your furniture) is a separate resource that can be attached to any VM (apartment) in the same region, resized without downtime, and even snapshotted for backups. The key mechanism is separation of concerns: managed disks decouple the virtual hard disk (VHD) from the virtual machine, allowing independent lifecycle management. Azure handles the underlying storage infrastructure (replication, encryption) just as the building management handles plumbing and electricity, so you only worry about your data.
What is an Azure Managed Disk?
An Azure Managed Disk is a block-level storage volume that is managed by Azure and attached to a virtual machine (VM). Think of it as a virtual hard disk (VHD) that you can use as the OS disk or data disk for your VM. The key innovation is that Azure handles the underlying storage infrastructure—the storage account, replication, and placement—so you don't have to. You simply create a disk, specify its size and performance tier, and attach it to a VM. The business problem it solves is the complexity and operational overhead of managing storage accounts for VM disks. Before managed disks, each VM disk required a dedicated storage account with specific performance limits, leading to capacity planning nightmares and throttling risks. Managed disks eliminate this by abstracting the storage layer.
How It Works: Step by Step
When you create a VM using the Azure portal or CLI, you are asked to choose a disk type for the OS disk. Behind the scenes, Azure creates a managed disk in the same region as the VM, places it in a storage cluster optimized for the selected performance tier, and configures replication (locally redundant storage by default). The disk is then attached to the VM as a SCSI device. The VM sees it as a local drive, but the data is actually stored across multiple nodes in the Azure datacenter for durability. When you write data to the disk, Azure replicates it synchronously within the datacenter (for LRS) or across zones (for ZRS). The disk can be detached from one VM and attached to another, as long as both VMs are in the same availability zone or region. This portability is a major advantage over unmanaged disks.
Key Components and Tiers
Azure Managed Disks come in several performance tiers: Ultra Disk, Premium SSD v2, Premium SSD, Standard SSD, and Standard HDD. Each tier offers different IOPS and throughput limits, and the cost scales accordingly. Ultra Disk provides the highest performance (up to 160,000 IOPS and 2,000 MB/s throughput) and is ideal for data-intensive workloads like SAP HANA or large databases. Premium SSD v2 is the next generation, offering lower latency and higher IOPS per GB than Premium SSD. Premium SSD is suitable for production workloads requiring consistent low latency. Standard SSD is a cost-effective option for dev/test or web servers. Standard HDD is the cheapest, for backup or infrequently accessed data. Additionally, there are two types of disks based on usage: OS disks (which contain the operating system) and data disks (for application data). Each VM can have multiple data disks, up to the VM size limit.
Pricing Models
You are billed for managed disks based on the provisioned size (not the actual data stored), the performance tier, and the number of transactions (for Standard HDD). For example, a 128 GB Premium SSD disk costs more than a 128 GB Standard HDD disk, even if both are empty. You can also use reserved capacity to save costs by committing to a one- or three-year term. Snapshots and disk images incur additional storage costs. The exam often tests the fact that you pay for the provisioned capacity, not used capacity, and that you can change the performance tier without downtime (though it may require a brief disconnection for some tiers).
Comparison to On-Premises Equivalent
On-premises, you would buy physical hard drives (HDDs or SSDs) and install them in a server. You would need to manage RAID configurations, monitor drive health, replace failed drives, and plan for capacity. Azure Managed Disks eliminate this hardware management. Instead, you provision a disk with the desired size and performance, and Azure ensures durability (99.999% SLA for Premium SSD) by replicating data across multiple servers. You never see the physical hardware. The trade-off is that you have less control over the underlying storage infrastructure (e.g., you cannot choose the exact model of SSD). For the exam, remember that managed disks provide high availability without you having to configure anything—Azure automatically replicates data.
Azure Portal and CLI Touchpoints
In the Azure portal, you can create a managed disk by navigating to 'Disks' under 'Storage' and clicking 'Create'. You specify the subscription, resource group, disk name, region, availability zone (optional), source type (snapshot, blob, or none), size, performance tier, and encryption options. For CLI, use:
az disk create --resource-group MyResourceGroup --name MyDisk --size-gb 128 --sku Premium_LRSTo attach a disk to a VM:
az vm disk attach --resource-group MyResourceGroup --vm-name MyVM --name MyDiskThe portal also shows metrics like IOPS, throughput, and disk queue depth. You can resize a disk (increase size) without downtime, but you cannot decrease it. You can also change the performance tier (e.g., from Standard SSD to Premium SSD) with a brief disconnection (for some tiers). Snapshots can be taken via portal or CLI:
az snapshot create --resource-group MyResourceGroup --name MySnapshot --source MyDiskThese snapshots can be used to create new disks or VMs.
Concrete Business Scenarios
Consider a company running a SQL Server database on an Azure VM. They need high IOPS for transaction logs. They choose Premium SSD managed disks for the data and log files. If they outgrow the disk, they can increase the size or change to Premium SSD v2 without redeploying the VM. Another scenario: a dev team uses Standard SSD disks for their test VMs to keep costs low. When they promote to production, they take a snapshot of the disk and create a new VM with Premium SSD disks from that snapshot. This demonstrates the portability and flexibility of managed disks.
Limits and Defaults
By default, a new managed disk is created with LRS (locally redundant storage). You can choose ZRS (zone-redundant storage) for Premium SSD and Standard SSD in selected regions. The maximum disk size is 32 TiB for most tiers, but Ultra Disk supports up to 64 TiB. The maximum number of data disks per VM varies by VM size; for example, a Standard_D2s_v3 supports up to 8 data disks. These limits are important for exam questions about scalability.
Create a Managed Disk
In the Azure portal, go to 'Disks' and click 'Create'. Fill in the subscription, resource group, disk name, region, and availability zone (optional). Choose the source type: 'None' for an empty disk, 'Snapshot' from an existing snapshot, 'Storage blob' from a VHD file, or 'Image' from a custom image. Select the size in GB (e.g., 128 GB) and the performance tier (e.g., Premium SSD). Encryption is enabled by default with platform-managed keys. Click 'Review + create'. Behind the scenes, Azure allocates storage in a datacenter cluster optimized for the chosen tier, configures replication (LRS by default), and creates the disk as a resource in your resource group. The disk is now ready to attach.
Attach Disk to a VM
Navigate to your VM in the portal, go to 'Disks', click 'Attach existing disks', and select the managed disk you created. Choose the logical unit number (LUN) if attaching multiple disks. The disk is attached as a SCSI device; the VM OS sees it as a new disk. For Windows, you need to initialize and format the disk in Disk Management. For Linux, you need to partition and mount it. Azure does not automatically format the disk. The attachment is a metadata operation; the data stays in the storage cluster. You can attach up to the VM's maximum data disk limit. Detaching is similar; you can detach from one VM and attach to another, as long as both are in the same region.
Resize a Managed Disk
To increase the size of a managed disk, go to the disk resource in the portal, select 'Size + performance', and increase the value in GB. You cannot decrease the size. The resize can be done while the disk is attached to a running VM for most disk types (except Ultra Disk, which may require a brief detach). Behind the scenes, Azure extends the underlying storage allocation; the VM sees the new capacity after a rescan. For example, if you increase a 128 GB disk to 256 GB, the VM will see a 256 GB disk after you extend the partition in the OS. This is useful when you run out of space. Note that you are billed for the provisioned size, so increasing the size increases cost.
Change Performance Tier
You can change the performance tier of a managed disk without downtime for some tiers (e.g., from Standard SSD to Premium SSD) by using the portal or CLI. In the portal, go to 'Size + performance', select a new tier, and click 'Save'. For most tiers, the change is immediate and does not require detaching the disk. However, changing to or from Ultra Disk requires the VM to be deallocated. Behind the scenes, Azure migrates the disk to a different storage cluster optimized for the new tier. This may cause a brief I/O pause (a few seconds) but no data loss. This flexibility allows you to right-size performance as workload demands change.
Create a Snapshot
A snapshot is a point-in-time, read-only copy of a managed disk. To create one, go to the disk resource, select 'Create snapshot', specify the name, resource group, and storage type (Standard HDD by default). You can also use CLI: `az snapshot create --resource-group MyRG --name MySnapshot --source MyDisk`. Snapshots are incremental by default for Premium SSD and Standard SSD disks, meaning they only store changes since the last snapshot, reducing cost. Snapshots can be used to create new disks or VMs for disaster recovery or testing. They are stored in the same region as the source disk. Note that snapshots are billed based on the actual data size (incremental), not the full disk size.
Scenario 1: E-commerce Platform with Variable Load
An online retailer runs its product catalog on Azure VMs. During Black Friday, traffic spikes 100x. The VMs use Premium SSD managed disks for fast read/write. The operations team sets up auto-scaling to add VMs during peak times. Each new VM attaches a pre-configured managed disk (from a snapshot) that contains the latest product data. This is fast because managed disks can be created from snapshots in minutes. After the sale, they scale down and delete the extra disks. The cost is only for the provisioned disk size during the time they are attached. If they had used unmanaged disks, they would need to manage storage accounts and worry about throttling. The key takeaway: managed disks simplify scaling and cost management.
Scenario 2: Lift-and-Shift Migration for a Database
A financial services company migrates its on-premises SQL Server to Azure VMs. They use Azure Site Recovery to replicate the VMs. The on-premises disks are converted to managed disks in Azure. The DBA wants to ensure high IOPS for transaction logs. They choose Premium SSD v2 disks because they offer lower latency and higher IOPS per GB than standard Premium SSD. During migration, they take a snapshot of the data disk and create a test VM to validate. Once validated, they cut over to the new VM. The managed disks support up to 32 TiB, so they can handle large databases. If they had misconfigured the disk type (e.g., choosing Standard HDD), the database would suffer from high latency and poor performance. This scenario highlights the importance of selecting the right disk tier for the workload.
Scenario 3: Disaster Recovery with Cross-Region Snapshots
A media company stores large video files on Azure VMs with Standard SSD disks. They need a disaster recovery plan. They regularly take snapshots of their data disks and copy those snapshots to a secondary region using Azure CLI or Azure Site Recovery. If the primary region fails, they can create new VMs in the secondary region from the snapshots. Managed disks make this easy because snapshots are incremental and can be copied across regions. The cost is the storage of snapshots and the data transfer. If they had not set up cross-region snapshots, a regional outage would cause data loss. The exam tests the concept that snapshots are regional, but you can copy them to another region manually.
Objective 2.4: Describe Azure storage services
AZ-900 questions on managed disks typically appear under this objective. The exam expects you to know the differences between managed and unmanaged disks, the performance tiers (Ultra Disk, Premium SSD v2, Premium SSD, Standard SSD, Standard HDD), the SLA (99.999% for Premium SSD), and key features like snapshots, encryption, and resizing.
Common Wrong Answers and Why Candidates Choose Them
'Managed disks require a storage account.' Candidates confuse managed disks with unmanaged disks. Unmanaged disks require a storage account; managed disks do not—Azure manages the storage account behind the scenes. This is a classic trap.
'You can decrease the size of a managed disk.' This is false. You can only increase the size. Candidates often think of on-premises disk shrinking, but Azure does not support it.
'Standard HDD provides the same performance as Standard SSD.' Candidates might assume all 'Standard' tiers are similar. In reality, Standard HDD is magnetic spinning disk (low IOPS), while Standard SSD is solid-state (higher IOPS). The exam tests this distinction.
'Managed disks are only for OS disks.' Managed disks can be OS or data disks. Candidates may think data disks must be unmanaged, but that is incorrect.
Specific Terms and Values
SLA: 99.999% for Premium SSD (single-instance VM with Premium SSD disks).
Maximum disk size: 32 TiB for most tiers, 64 TiB for Ultra Disk.
Replication: LRS by default; ZRS available for Premium SSD and Standard SSD in selected regions.
Encryption: Azure SSE (Storage Service Encryption) is enabled by default with platform-managed keys; customer-managed keys are optional.
Snapshots: Incremental by default for Premium SSD and Standard SSD; full copy for Standard HDD.
Edge Cases and Tricky Distinctions
Ultra Disk vs. Premium SSD v2: Ultra Disk offers the highest IOPS but is not compatible with all VM series (e.g., requires a VM that supports Ultra Disk). Premium SSD v2 is more broadly compatible.
Host caching: Managed disks support host caching (ReadOnly, ReadWrite) to improve performance. The exam may ask about caching options for OS vs. data disks.
Shared disks: Some managed disks can be shared across multiple VMs (for clustered applications), but this is a premium feature.
Memory Trick
Use the acronym 'PUSH' for Premium SSD: Premium Ultra Standard HDD (from best to worst). Or remember: 'Ultra - Very Fast, Premium - Fast, Standard SSD - Good, Standard HDD - Slow.' For the decision tree: If you need max performance, choose Ultra Disk (if supported) or Premium SSD v2. For production workloads, Premium SSD. For dev/test, Standard SSD. For backup, Standard HDD.
Azure Managed Disks are block-level storage volumes that are managed by Azure, eliminating the need to handle storage accounts.
There are five performance tiers: Ultra Disk, Premium SSD v2, Premium SSD, Standard SSD, and Standard HDD, each with different IOPS/throughput and cost.
You are billed for the provisioned size of the disk, not the actual data stored.
You can increase the size of a managed disk without downtime, but you cannot decrease it.
Snapshots of managed disks are incremental by default (for Premium and Standard SSD), reducing storage costs.
The default replication is Locally Redundant Storage (LRS); Zone-Redundant Storage (ZRS) is available for Premium SSD and Standard SSD in select regions.
Managed disks support encryption at rest with platform-managed keys by default; customer-managed keys are optional.
These come up on the exam all the time. Here's how to tell them apart.
Managed Disks
Azure manages the storage account; no manual configuration.
Supports resizing without downtime (increase only).
Snapshots are incremental by default for Premium/Standard SSD.
Encryption at rest enabled by default (SSE).
Higher SLA (99.999% for Premium SSD).
Unmanaged Disks
Requires you to create and manage a storage account.
Resizing requires deleting and recreating the disk.
Snapshots are full copies (no incremental).
Encryption must be configured separately.
Lower SLA (99.9% for storage account).
Mistake
Managed disks require you to create a storage account first.
Correct
No, managed disks abstract the storage account. Azure automatically manages the underlying storage infrastructure. You simply create the disk resource.
Mistake
You can shrink a managed disk to reduce cost.
Correct
You cannot decrease the size of a managed disk. You can only increase it. To reduce size, you must create a new smaller disk and copy data.
Mistake
All managed disks use the same replication type by default.
Correct
The default replication is Locally Redundant Storage (LRS). Zone-Redundant Storage (ZRS) is available only for Premium SSD and Standard SSD in selected regions and must be explicitly chosen.
Mistake
Managed disks are only for data disks, not OS disks.
Correct
Managed disks can be used for both OS and data disks. When you create a VM, the OS disk is automatically a managed disk unless you choose unmanaged.
Mistake
You cannot attach a managed disk to multiple VMs.
Correct
By default, a managed disk can only be attached to one VM. However, Azure offers shared disks (for Premium SSD and Ultra Disk) that can be attached to multiple VMs in a cluster, but this is a special feature.
Managed disks are Azure-managed storage volumes that do not require you to create a storage account. Azure handles the underlying storage infrastructure, replication, and placement. Unmanaged disks require you to create a storage account and manage the VHD files within blob containers. Managed disks offer higher SLA (99.999% for Premium SSD vs. 99.9% for unmanaged), support incremental snapshots, and allow resizing without downtime. For AZ-900, remember that managed disks are the modern recommended approach.
For most tiers (e.g., Standard SSD to Premium SSD), you can change the performance tier without downtime. The change may cause a brief I/O pause (a few seconds) but no data loss. However, changing to or from Ultra Disk requires the VM to be deallocated. Always check the documentation for specific tier transitions. This is a key exam point: you can change tiers without redeploying the VM.
The maximum size for most managed disk tiers (Premium SSD v2, Premium SSD, Standard SSD, Standard HDD) is 32 TiB. Ultra Disk supports up to 64 TiB. The maximum size is important for exam questions about scalability. You can also stripe multiple disks together using Storage Spaces or RAID to achieve larger volumes.
You can take a snapshot via the Azure portal by selecting the disk and clicking 'Create snapshot', or via CLI with `az snapshot create`. Snapshots are incremental by default for Premium SSD and Standard SSD disks, meaning they only store changes since the last snapshot. Snapshots are stored as managed disks and billed based on actual data size. They can be used to create new disks or VMs.
The SLA for a single-instance VM using Premium SSD managed disks is 99.9% for connectivity and 99.999% for disk durability (data is replicated across three copies in the datacenter). For multiple-instance VMs with Premium SSD, the SLA is 99.95% for connectivity. The exam often asks about the 99.999% durability figure.
By default, a managed disk can only be attached to one VM at a time. However, Azure offers shared disks (available for Premium SSD and Ultra Disk) that can be attached to multiple VMs in a clustered configuration (e.g., SQL Server FCI). Shared disks have specific limitations and are not covered in depth on AZ-900, but you should know that regular managed disks are single-attach.
Standard SSD uses solid-state drives, offering higher IOPS (up to 6,000) and lower latency than Standard HDD, which uses magnetic spinning disks (up to 2,000 IOPS). Standard SSD is suitable for production workloads that need consistent performance, while Standard HDD is for backup, dev/test, or infrequently accessed data. Both are cheaper than Premium SSD.
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