Skipping Swap File Disks (Citrix XenServer)
If you place your swap file data for each virtual machine on a separate virtual disk, you can use the PHDVB:skip tag to skip this disk each time the VM is backed up.
Skipping the virtual disks that contain swap file data is a good idea because you do not need to save this data. Also, backup compression and deduplication ratios for your VMs will improve because swap file disks contain effectively random data that does not compress or deduplicate well.
Additional details about using skip tags are included in the documentation.