Clarify when certain disk operations entail a background copy of data

Noah S 121 Reputation points
2024-09-19T17:17:59.6+00:00

Microsoft docs say the following about when the background copy process is invoked for disks:

The following control plane operations might move the disk between storage locations, and cause increased latency:

  • Update the storage type.
  • Detach and attach a disk from one VM to another.
  • Create a managed disk from a VHD.
  • Create a managed disk from a snapshot.
  • Convert unmanaged disks to managed disks.

Can we predict when detaching and attaching a disk from one VM to another involves a background copy? I have a case where I want to avoid the performance penalty of the background copy. Does using VMs in the same region/availability zone guarantee that disks can be moved between VMs without the disk being copied in this way?

Azure Disk Storage
Azure Disk Storage
A high-performance, durable block storage designed to be used with Azure Virtual Machines and Azure VMware Solution.
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  1. KarishmaTiwari-MSFT 20,037 Reputation points Microsoft Employee
    2024-09-19T21:38:16.1233333+00:00

    @Noah S Thanks for posting your query on Microsoft Q&A.

    If both VMs are in the same region and availability zone, the likelihood of a background copy is reduced, but not entirely eliminated. The disk might still be moved between different storage locations within the same region, which can trigger a background copy.

    Changing the storage type (e.g., from Standard HDD to Premium SSD) will likely involve a background copy, regardless of the region or availability zone.

    For managed disks, Azure handles the storage location and type automatically. If the managed disk needs to be moved to a different storage location for any reason, a background copy might be triggered.


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