Hi there,
DPM uses loopback-mounted-VHDs. These appear like normal disks to the OS. Therefore, these disks are displayed in Windows Explorer, Diskmgt, and other GUI tools. These tools periodically poll the disks to make sure that they are functioning correctly.
This causes IOs to be sent down the loopback stack to the ReFS volume. If the ReFS volume is busy, these IOs will have to wait. Therefore, when ReFS performs a long-duration operation, such as flushing or a large block-clone call, these IOs will have to wait longer.
Here are some links to help you out
ReFS volume using DPM becomes unresponsive on Windows Server 2016
https://learn.microsoft.com/en-us/troubleshoot/windows-server/backup-and-storage/refs-volume-dpm-unresponsive
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