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What is Disk in Azure?



What is Disk in Azure?

Definition: In Azure, Disk is a hard disk attached to a VM (Virtual Machine).

Virtual Disks are stored as Page BLOBs in Azure.

What are the Disks used for?

  • OS Disk: One disk is used in each Virtual Machine to hold operating system files. When you select a VM image while creating it, that sets up OS and OS disk associated with that new VM. The OS disk has a maximum of 4TB of capacity.
  • Data Disk: You can add one or more virtual data disks to store the data like files, images, videos, games, etc... The number of data disks depends on the size of the VM.
  • Temporary Disk: Each VM has one Temporary disk. This is used for short-term storage applications like page files, swap files. These disks are local to the server and are not stored in the Storage account. This disk cannot be used for critical data as in the maintenance activities, temporary disk content will be lost.

What is an Ephemeral OS disk in Azure?

  • Definition: Ephemeral OS disks are a kind of stateless disk storage.
  • Ephemeral OS disks are created on the local VM and not on the remote storage.
  • They have faster latency than managed disks.
  • They are stored locally so that’s why they are free.
  • It is very fast to reset or reimage the VM to its original boot state.
  • Individual VM failure may destroy all the data on the Ephemeral disk leaving VM unable to boot.
  • This type of disk works well with stateless applications.
  • Available in all the Azure regions.

Disk Performance Measures

Disk performance can be measured by two parameters.

  • Input/output operations per second (IOPS): This is the rate that tells you how many read/write operations can be performed by disk in a second. The higher the rate, the better the performance.
  • Throughput: This is the value that tells you the rate at which the data is being transferred from the disk to the host computer and from the host computer to a disk. In simple terms, this is the data transfer rate. It is measured in Mbps. Higher performance disks have higher throughput rates.

There are two types of disks. Managed and Unmanaged.

Managed Disk vs Unmanaged Disk

Managed Disk:

  • Most of the disks we use in Azure are Managed disks
  • Azure manages everything in the Managed disk and you just need to provision them and attach them to VM, so they are easier to use.
  • The managed disk provides scalability, High Availability, Backup, Data Encryption, Role-based access control (RBAC) on a disk to perform a specific task (like delete can be performed only by the Admin).
  • You don’t need to create a Storage account, BLOB Container, or Page BLOB yourself

Unmanaged Disk:

  • You need to create and manage the storage account manually by yourself.
  • You need to keep monitoring the IOPS (Input/Output Operations per Second) limit within the storage account.
  • You need to manage security and RBAC access at the Storage account level instead of handling it at each disk level.
  • Unmanaged Disk doesn’t support scalability so you need to keep an eye on the usage of the disk, and you will have to scale up/down manually.

What are the types of Disk in terms of storage?

  • Standard HDD: Standard Hard Disk Drive.
    • This is low in cost.
    • You can use it for scenarios like backup, Development, or Testing.
    • This can be used for less critical workloads
  • Standard SSD: Standard Solid-State Drive.
    • This gives you better performance compared to HDD.
    • Along with that, it gives better reliability and scalability.
    • Use this when you have budget issues.
    • Like HDD, this can be used for Dev or test environments.
  • Premium SSD:
    • As the name suggests, Premium SSD is better than both standard SSD and HDD disks.
    • They are a bit costlier than the above both (SSD, HDD) disks, but the performance is guaranteed unless impacted because of a few occasional performance drops.
    • This can be used on medium and large organizations in critical scenarios.
    • You can’t adjust the performance (Throughput and IOPS) without detaching the disk drive from VM.
  • Ultra Disk:
    • They provide the highest performance among all types of disks.
    • You can choose this when the application is very critical, and you need the fastest storage performance.
    • It provides the Highest throughput, Low latency, and high IOPS.
    • You can adjust performance (Throughput and IOPS) while they are running (without detaching). An update may take some time though.
    • There are some limitations of the Ultra disk.
      • They can be attached to the VMs only in some of the regions. Basically, not available right now across the globe.
      • Can be used as a data disk and can be created as an empty disk.
      • Don’t support disk encryption, azure backup, azure site recovery, disk snapshot, or VM images.


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