In HotAdd transport, virtual disks on backed-up VMs are HotAdded to the backup proxy so they can be easily saved to backup media.
Deploy the proxy on VMFS-6 or VMFS-5 volumes, so that the proxy can back up very large virtual disks. HotAdd is a SCSI feature and does not work for IDE disks. The paravirtual SCSI controller (PVSCSI) is the recommended default for HotAdd, but other controller types work too. You might want to include checks in your code to detect that your HotAdd appliance has controllers in the correct order.
The HotAdd implementation assumes that proxy can access (connect to) target VMs. This is so VADP can obtain a list of all disks on target VMs from the connection.
A redo log is created for HotAdded disks, on the same datastore as the base disks. Do not remove the target virtual machine (the one being backed up) while HotAdded disk is still attached. If removed, HotAdd fails to properly clean up redo logs so virtual disks must be removed manually from the backup appliance. Also, do not remove the snapshot until after cleanup. Removing it could result in an unconsolidated redo log.
HotAdded disks should be released with VixDiskLib_Cleanup() before snapshot delete. Cleanup might cause improper removal of the change tracking (ctk) file. You can fix it by power cycling the virtual machine.
Virtual disk created on Windows by HotAdd backup or restore may have a different disk signature than the original virtual disk. The workaround is to reread or rewrite the first disk sector in NBDSSL mode. Customers running a Windows Server 2008 or later proxy should make sure Windows automount is deactivated.
For unbuffered HotAdd restore, VMware recommends that programmers set the VDDK flag VIXDISKLIB_FLAG_OPEN_UNBUFFERED when opening virtual disks before performing a restore with HotAdd transport. In vSphere 6.7 releases and later, programs must allocate a data buffer whose memory address is sector size aligned when setting this flag. Programmers may refer to posix_memalign on Linux or _aligned_malloc on Windows. Buffer address alignment to sector size is recommended for older VDDK releases as well.
In the rare case that VixDiskLib_Close() fails with error VIX_E_HOST_SERVER_NOT_AVAILABLE in HotAdd transport mode, the backup application should run VixDiskLib_Cleanup() and notify the administrator to manually remove the snapshot after the host exits maintenance mode, then retry the backup.
