Post by Abdul Rasheed (thank you)
When you backup a VMware vSphere virtual machine using vStorage APIs for Data Protection (VADP), one of the common ways to transmit data from VMware data store to backup server is through Network Block Device (NBD) transport. NBD is a Linux-like module that attaches to VMkernel and makes the snapshot of the virtual machine visible to backup server as if the snapshot is a block device on network. While NBD is quite popular and easy to implement, it is also the least understood transport mechanisms in VADP based backups.
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