DPM 2016 Restore RCT Hyper-V VM backup size incorrect? RRS feed

  • Question

  • Hello all,

    we use DPM 2016 rollup 6 to protect a Hyper-V cluster with Windows Server 2016 nodes. The backups show as RCT backups.

    We have one VM that has a 300 GB fixed virtual hard drive (VHDX). I did a restore of that VM to a network folder. The VHDX file from the restore is only a little over 200 GB in size. Should i be worried about the difference in file size? 300 GB production VM to 200 GB restored file size seems strange. Is there some Hyper-V RCT going on or what?

    Thursday, April 11, 2019 9:12 AM

All replies

  • Hello Christian,

    The main difference is probably the allocation unit size of the disks, DPM 2016 and newer uses ReFS to store it's backups, the ReFS disks uses by default the allocation unit size of 64K, while NTFS disks uses 4K by default.

    The allocation unit size represents the smallest amount of disk space that can be allocated to hold a file. 
    Because ReFS and NTFS don’t reference files at a byte granularity, the allocation unit size is the smallest unit of size that each file system can reference when accessing storage.

    Another thing would be if the folder where the VM was restored to has compression or data deduplication.

    Best regards,

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    Thursday, April 11, 2019 9:48 AM
  • The VM uses a static VHDX. The restorted VM uses a dynamic VHDX with 220 GB used of 300 GB. No idea on what and why that happens.

    Thursday, April 11, 2019 11:16 AM
  • Did you just restore a VHDX or a whole VM?

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    Thursday, April 11, 2019 11:57 AM