For each new replication that you configure, an initial full sync operation is performed. During initial full sync, vSphere Replication copies the whole data from the source virtual machine to a placeholder vApp on the target site.
If the source virtual machine is too big, or the bandwidth of your network connection to the cloud is too low, the initial full sync might take a long time. Therefore, you might choose to copy the source virtual machine to the target site by using removable media, or other means of data transfer. Then you can configure a replication and use the virtual machine copy on the target site as a replication seed. When a replication is configured to use a seed vApp, vSphere Replication does not copy the whole source virtual machine to the target site. Instead, it copies to the seed vApp only the different blocks between the source virtual machine and the seed.
vSphere Replication stores the replication data in the seed vApp. No copies of the seed vApp are created. Therefore, a seed vApp can be used for only one replication.
Creating Seed vApps in the Cloud
Seed vApps on the target site can be created in the following ways.
Offline data transfer: You can export a virtual machine as an OVF package and let a vCloud Hybrid Service administrator import the package in your cloud organization.
Clone a virtual machine: A virtual machine in the org virtual data center can be cloned to create a seed vApp. vSphere Replication calculates checksum and exchanges the different blocks from the replication source to the seed vApp.
Copy over the network: A source virtual machine can be copied to the cloud organization by using means other than vSphere Replication to copy the initial source data to the target site.
The size and number of disks, and their assignment to disk controllers and bus nodes must match between the replication source and the seed virtual machine. For example, if the replication source machine has two disks of 2 GB each, one of them assigned to SCSI controller 0 at bus number 0, and the second one assigned to SCSI controller 1 at bus number 2, the seed vApp that you use must have exactly the same hardware configuration - 2 disks of 2 GB each, at SCSI 0:0 and at SCSI 1:2.