No XenServer, o processo é usar o XenConvert. (Você converte com um HDD encolhido e reimporta a imagem no XenServer)
Por favor, veja essa KB. Como redimensionar um disco virtual XenServer
A KB foi criada em 2014, mas no antigo XenServer essa dica funcionou. O estado da ponta para usar o XenConvert 2+, mas em 2011 já foi lançado.
Texto da KB:
To complete the following procedure, you need the access to Access to XenConvert 2.x.
1.Download and install XenConvert on the Windows VM.
2.Attach a secondary disk that is 205 percent times the size of the existing VDI. The secondary drive is to be used as temporary space for XenConvert.
3.Format the secondary disk with an NTFS file system and add a drive letter to the disk.
4.Open XenConvert and click Next.
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5.Adjust the free space size as the following:
•Example for original size: 14406
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•Example for New size: 5000
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Description of the fields in the preceding figures:
•Used Space: The space currently being used in the VDI.
•Free Space: The new size for the VDI. So if you want to make it 10GB, you change the value to 10240 MB.
•Unallocated space: The additional space you give to the VM and that is displayed as unallocated space.
•Capacity: The value in the last row is the VM VDI total size:
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Proceed with the XenConvert utility to create a disk to import later or to create the new VDI on the XenServer.