All the virtual machine data is eventually stored in magnetic disks. The only difference is that some are written immediately and some at a later point in time sequentially.
Let's review specific I/O workflows that are associated with VSAN:
Write buffer
Data destage
Read cache
A guest operating system within the virtual machine performs a write operation intended to the virtual disk associated to the virtual machine. The I/O is received by the Virtual SAN module on the localhost of the virtual machine that it is currently running on. On receipt of the I/O, the VSAN knows the replicas of the virtual disk and triggers a parallel write operation to the replicas. The writes are performed first on the flash devices that frontend the magnetic disks holding the replicas. Once the I/O completes on all the flash devices, an acknowledgement is sent back to the guest OS that the I/O is complete.
The data written on the flash device is eventually retired/destaged to the final destination...