Throughput for backups running against ESXi (4.1) or vSphere 5 where the VMware datastores are part of an iSCSI SAN, and are connected to the VMWare ESXi hosts via the VMWare Software iSCSI adapter, appear to be too slow. LAN-mode ESXi/vSphere 5 backups always will be slower than SAN-based or Hot-Add backups, but no matter the type of backup in use, the throughput for the backup is low.
There are many possible causes to this issue (it's a complex topic), but since the VMware Software iSCSI adapter is in use in this example,(i.e.if the ESXi/vSphere 5 hosts have no hardware iSCSI HBAs (Host Bus Adapters) installed in them), the task of the disk I/O for reading from/writing to the VMware datastores is handled via software (the vmkernel). Since there is no hardware acceleration present, this can induce a large amount of CPU overhead on the ESXi/vSphere 5 hosts, slowing-down overall performance.
It has been demonstrated that (if possible),when creating VMs on such an environment, the VMware paravitualized NIC should be used (vmxnet3 adapter) rather than the default "E1000" adapter on operating systems that support it. This paravirtualized device provides better resource management on the host-side by using better offloading, using better CPU management, etc. If VMs (must be at least version 7 Virtual Hardware) on a given ESXi/vSphere5 host have this NIC configured instead of the less efficient non-paravirtualized NICs, the CPU load on the host itself will lessen, allowing the VMWare vStorage APIs used by vRanger to benefit from the better software I/O management. This may be especially apparent if the vRanger machine is hosted on a VM, and the vmxnet3 paravirtualized NIC is in use for its connections as well. The reason for this is that the ESXi/vSphere 5 backups all proxy through and are processed by the vRanger machine, so it not only benefits from better read i/o from the ESXi/vSphere 5 datastores, but it's own read/writes are also boosted by the more modern paravirtualized NIC (especially useful if the vRanger machine is writing the backups out through the virtual network interface to a NAS device or remote network share).
There are caveats to what operating system support the Paravirtualized NIC, and please consult the following two VMWare KB articles for best-practices information and compatibility:
http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&exter
http://www.vmware.com/pdf/vsp_4_vmxnet3_perf.pdf
