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.
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.
Removing all disks on a controller with the vSphere Client also removes the controller. You might want to include some checks in your code to detect this in your appliance, and reconfigure to add controllers back in.
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 disabled.
The HotAdd implementation assumes that proxy and target VMs are on the same datastore and accessible from the same connection, that is, the same vCenter Server. This is so VADP can obtain a list of all disks on the target VM from a connection. In vCloud environments where two vCenter Servers share a single datastore, causing VADP to make two connections, there is no mechanism for informing one connection of the disks available on the other connection. VMware needs to redesign HotAdd to support multiple connections.
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.
