Best Practices for
HotAdd Transport
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.