This feature is to support VSS (Volume Shadow Copy Service) on Hyper-V.
VSS is a service on Hyper-V. Any VM wants to support this feature should implement such service on VM side.
When Hyper-V host wants to do a snapshot for a given FreeBSD VM, it sends request of freezing file system to VM. VM freezes its file system, and responses to Hyper-V. Then Hyper-V starts to take snapshot. After that, Hyper-V will inform VM to thaw the file system.
On FreeBSD side, the VSS driver implements the protocol to communicate with Hyper-V host and a user space daemon (hv_vss_daemon). The daemon hv_vss_daemon conducts the real UFS freeze/thaw.
Submitted by: Hongjiang Zhang <honzhan microsoft com>