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SCSI HotAdd is the default transport method for VMware's vSphere Data Protection tool, which connects the virtual disk file being backed up directly to the VDP appliance. This logical correlation keeps backup traffic on the local system and off the external network to reduce backup time.
However, HotAdd can pose problems under some conditions. For example, HotAdd only works with local SCSI disks; the HotAdd feature must be licensed for each vSphere host; the data store hosting the virtual machines must be accessible to the vSphere host running VDP; and there must be a free SCSI device available on the host -- supporting up to four SCSI controllers with 15 devices per controller or up to 60 SCSI disks on the host. If the environment cannot support HotAdd for any reason, VMware VDP can also be configured to use network block device or NBD with secure socket layer encryption.
When SCSI HotAdd is available, there are some additional configuration tactics to ensure best performance for VDP. First, HotAdd is intended exclusively for SCSI disks and will not operate with IDE-based storage devices because IDE suffers reduced I/O. The total capacity and block size of the VMFS volume where VDP is installed should be at least as large as the largest virtual disk being protected. For example, if you use VDP to protect a virtual disk on a data store with 1 MB blocks, the VDP host must also be on a data store with 1 MB blocks. Otherwise, HotAdd cannot be used and an alternate transport will need to be configured. Deploying VMware VDP on VMFS5 or later may help ease block-size limitations.
In addition, HotAdd cannot cross data-center boundaries, so the VM being protected must be within the same data center. This can pose potential problems for organizations that routinely migrate VMs across data center boundaries. Port configuration may be an issue. VDP needs access to port 902 on ESXi hosts when HotAdd is configured. If port 902 is closed or cannot be opened for security reasons, HotAdd may not be available and alternate transports should be selected.
Finally, HotAdd and vSphere Flash Read Cache are not compatible and will not work together; vFlash VMs may need alternate transports for VMware VDP support.
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