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Size Measures for Virtual Machines

Reports and job status dialog boxes show various size measures for virtual machines and backup jobs:

  • Provisioned size - The maximum configured disk size for a VM in the hypervisor.
  • Actual VM size - Total used space size of all disks on the VM datastore or volume.

    For VMs with dynamic disks (thin disks in VMware), the actual disk size can be less than the provisioned size. For fixed disks (thick disks in VMware), the actual disk size is the same as the provisioned size. If there are snapshots existing on the VM, actual space taken by all disks could be much more than the total provisioned disk sizes.

  • Backup Size - The amount of data backed up after eliminating white spaces. Data that was written and deleted still counts as reserved (allocated) space.
  • Size of Application - The sum of the backup sizes of each VM included in a Virtual Server Agent (VSA) backup job.
  • Guest size - The guest size is determined by comparing the following calculations, and using the smallest amount as the guest size for the VM's capacity usage.
    • Allocated size
    • Logical Volume Manager (LVM) processing size
    • Provisioned size
    • VMware Tools size
    • Backup size

      Note: Guest size is only calculated for NTFS volumes.

For example:

  • If a virtual machine has a single dynamic disk that is provisioned for 100 GB but only uses 20 GB, the provisioned or allocated size is 100 GB. With this, if the size data obtained via the VMware tools is 20 GB and the backup size is 30 GB, then the guest size is likely to be close to 20 GB.
  • If a virtual machine has a single fixed disk of 100 GB but only 20 GB of data has been written to it, the provisioned size and actual VM size are 100 GB, With this, if the size data obtained via the VMware tools is 20 GB and the backup size is 30 GB, then the guest size is likely to be close to 20 GB.
  • If a virtual machine has a single fixed disk of 100 GB but only 20 GB of data has been written to it, the provisioned size and actual VM size are 100 GB. Furthermore, if the size data could not be obtained via the VMware tools and the backup size is 30 GB, then the guest size for this VM is likely to be close to 30 GB.

The following table shows examples of how size measures vary with dynamic and fixed disks.

VMs

Disks

Size Measures

VM Alpha

2 dynamic disks with 10 GB each

5 GB of disk space used on each disk

4 GB of data currently on each disk

Provisioned size = 20 GB

Actual VM size = 10 GB

Guest size = 8 GB

Backup size = 8 GB

VM Beta

2 fixed disks with 10 GB each

10 GB of allocated space on each disk

4 GB of data currently on each disk

Provisioned size = 20 GB

Actual VM size = 20 GB

Guest size = 8 GB

Backup size = 8 GB

 

 

Size of application for both VMs = 16 GB

Depending on the hypervisor, some of these size measures might not be applicable.

Size Information for Jobs and Reports

For specific backup jobs, the Job Details dialog box shows the backup size and guest size for the job. These values can be different from each other, and both can be different from the used space value shown in the disk properties dialog by Microsoft Windows. The layering effects of multiple virtual file systems can cause differences between the size reported by the guest host running within the VM and the reported backup size. Frequent deletion of large files can easily cause these numbers to be out of sync.

The Backup Job Summary Report includes several size measures, including guest size, backup size, and size of application.

Factors That Can Affect the Backup Size Calculation

  • The presence of virtual machine snapshots.
  • The presence of sparse files or deleted files in the guest can cause the backup size to vary.
  • The backup size reported for virtual machine disks in a VM can vary depending on white space and change allocation tracking for that VM.
  • By default, page and swap files are skipped during backups for VMware and Hyper-V.

    If VMware Tools are not installed on VMware guest VMs, or if Hyper-V Integration Services are not installed on Hyper-V guest VMs, page and swap files are included in backups.

  • Application size is the backup size across VMs, eliminating any white spaces (blocks of zeros). For sparse files this size may be less than the reported backup size.

Measures to Reduce Application Size

  • Delete or move unnecessary data before virtualizing physical machines. This saves resources and time, and ensures that new VMs only contain used blocks.
  • If you already have a large number of VMs with significant reserved and unused blocks, use a tool such as the Windows SDelete utility to release reserved space on VMs, as described in SDelete v1.61.
  • For VMware, review the following factors:
    • Ensure that VM templates used for provisioning do not contain unreferenced blocks in the disk.
    • Wherever possible, configure virtual machine disks with dynamic provisioning. Dynamic provisioning ensures only valid blocks occupy space in the disk file.  With dynamic provisioned disks, data protection APIs only return occupied blocks.

Last modified: 3/25/2019 7:39:58 PM