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Knowledge Base

VMware Consolidated Backup SCSI Hot-Add feature (1008072)

Last Updated: 5/14/2010Categories: Total Views: 11985Language:
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Details

With VMware Consolidated Backup 1.5, VMware has introduced a new transport mode called “SCSI Hot-Add”. This article supplements the Virtual Machine Backup Guide to explain this feature.

Solution

What is the SCSI Hot-Add feature?

With SCSI Hot-Add, you set up one of your virtual machines as a VCB proxy and use it to back up other virtual machines residing on storage visible to the ESX Server that hosts the VCB proxy virtual machine.

Using hot-add should be faster option to perform backups than LAN mode, since it mounts disks from virtual machines via Shadow Proxy Virtual Machine.

What are the components needed to use SCSI Hot-Add?

The components are:

A Windows virtual machine which meets the requirements to run as a VCB proxy (see the Virtual Machine Backup Guide for supported Windows versions)

A (shadow) Helper virtual machine

Are there any special configurations that one needs to setup to use this feature?

There are few things to take into consideration about the VCB proxy and the Helper virtual machine before implementing the Hot-Add functionality:

Location of the virtual machine – The Helper virtual machine (or) Proxy virtual machine can be located on any datastore inside the ESX server, provided the ESX host:

Can access the datastore hosting the virtual machines (VCB proxy and Helper virtual machine)

Can access to the datastores hosting all the virtual machines that need to be backed up

Helper virtual machine specific

The Helper virtual machine should never be powered on.

The name of the virtual machine. For example, if the VCB Proxy virtual machine is named vcb-15, the VCB Helper virtual machine should be named vcb-15(VCB-HELPER). For more information on naming the Helper virtual machine, see Creating a VCB Helper Virtual Machine in the Virtual Machine Backup Guide.

How does VCB SCSI hot-add work?

Here is a high-level workflow of how the feature works:

Third party backup software (if present) initiates a virtual machine backup job using the VCB framework

VCB framework executes vcbmounter.exe with –m hotadd option

Creates linked clones of the VMDKs associated with the virtual machine to be backed up to the helper virtual machine

Reload the helper virtual machine with the attached linked cloned VMDKs.

Create a linked clone to a uuid folder within helper virtual machine directory

Attach cloned disks to the proxy virtual machine

Rescan scsi bus and create stub VMDK files on the Proxy virtual machine

Perform backup operation

Unmount disks from the VCB Proxy virtual machine

Remove/delete clone disks (leaves behind vmx, nvram, vmsd files) on the VCB Proxy virtual machine

Steps needed to setup VCB 1.5 for hotadd transport

Create a shadow virtual machine with the same name as VCB proxy and add VCB-HELPER to the name.
The name must be similar to:

VCB_NAME(VCB-HELPER)

Put this virtual machine on a shared vmfs volume that is accessible from all ESX hosts.
Note: This virtual machine does not require a disk. Also, it does not have any special CPU/Memory requirements. Ensure, that it is powered off.

Run vcbMounter with option -m hotadd

During the backup, a Snapshot is created for the virtual machine which is being backed up. Inside the VCB, a shadow vm directory hash is created for each virtual machine. For example, 500779ca8f5d7d7ce0a2ec17beea451b is a shadown virtual machine directory.

The directory contains the following files:

[root@hostname VC2.5U2(VCB-HELPER)]# ls -l 500779ca8f5d7d7ce0a2ec17beea451b

total 192
-rw------- 1 root root 8684 Jul 28 18:46 500779ca8f5d7d7ce0a2ec17beea451b.nvram
-rw------- 1 root root 51 Jul 28 18:46 500779ca8f5d7d7ce0a2ec17beea451b.vmsd
-rwxr-xr-x 1 root root 2828 Jul 28 18:46 500779ca8f5d7d7ce0a2ec17beea451b.vmx

It also creates linked clones:

-rw------- 1 root root 10752 Jul 29 2008 win4test-cl1-delta.vmdk
-rw------- 1 root root 280 Jul 29 2008 win4test-cl1.vmdk

which is pointing to vm basic disk and is attached to VCB proxy, during the export time period:
scsi0:1.deviceType = "scsi-hardDisk"
scsi0:1.filename = "/vmfs/volumes/48888777-64e1cf76-4ccf-00151725d4ca/VC2.5U2(VCB-HELPER)/500779ca8f5d7d7ce0a2ec17beea451b/win4test-cl1.vmdk"

Also it creates directory VmdkStubs for vcbMounter export time (for instance C:\mnt\test91\VmdkStubs).

If VCB shadow virtual machine is not created on shared vmfs, vcbMounter may fail with following error:

[2008-07-29 08:43:14.156 'vcbMounter' 2040 error] Error: The managed object of type "vim.VirtualMachine" identified by the MoRef "vm-104" could not be found.

Detectable by VMware SkylineTM

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