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Nuts and bolts in NetBackup for VMware: Avoiding CBT penalty with NetBackup Accelerator

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Employee Accredited Certified

Let us start our technical deep dive with NetBackup Accelerator for VMware. Most of you already know that NetBackup Accelerator is designed to provide full backups for the cost of performing an incremental backup. In this case cost stands for the backup window, backup storage, client CPU, client memory, client disk I/O, network bandwidth etc. required for running backups. NetBackup Accelerator was first introduced in NetBackup 7.5 for file system workloads and became an instant hit. Answers to frequently asked questions on NetBackup Accelerator for files systems could be found here.

In NetBackup 7.6, backup acceleration comes to both VMware vSphere and VMware vCloud Director environments (more on VMware vCloud Director support in a later blog in this series). NetBackup Accelerator for VMware is a combination of three powerful technologies.


  1. Changed Block Tracking in VMware vStorage APIs for Data Protection: The change block tracking (CBT) capability in vSphere lets data protection applications like NetBackup to retrieve disk blocks from virtual machine disk file (VMDK file) that has changed since a previous pre-assigned point in time. NetBackup had supported CBT for incremental backups ever since VMware introduced that capability in vSphere 4.0. NetBackup Accelerator makes use of CBT to detect changed blocks.
  2. Symantec V-Ray: V-Ray for virtual machine is like how X-Ray is for a physical object. It is a patented Symantec technology that enables NetBackup to see into virtual machine disk files without mounting them or secretly pushing obscure agent binaries into the virtual machine. NetBackup Accelerator for VMware uses this capability to enumerate underlying files/application objects in the changed blocks. It addresses one of the weaknesses in change block tracking, detecting deleted files. 
  3. NetBackup Optimized Synthetics: NetBackup Optimized Synthetics creates full backup images without reading and shipping entire data set from source to backup storage. The changed blocks detected through VMware CBT is passed through Symantec V-Ray lens for optimization and then injected into Symantec’s Optimized Synthetics capable deduplication engine to create a full recovery point.


Just to demonstrate the power of NetBackup Accelerator, let me walk you through the process with an example. Imagine that you have a production Microsoft SQL Server database in a Microsoft Windows virtual machine. Let us say that you have three VMDKs in this virtual machine.

  • VMDK 1: This is C:\ with operating system and application binaries installed.  100GB capacity. 60GB is occupied.
  • VMDK 2: This is where Microsoft SQL Data files are present. 500GB capacity. 250GB occupied.
  • VMDK 3: Microsoft SQL Server logs are located here. 100GB capacity. 20GB occupied.

During the first backup, VMware CBT has nothing to offer. All blocks from VMDK files must be processed. Thanks to Symantec V-Ray, NetBackup sees into the data stream. Hence whitespaces are eliminated from being read and moved. The total capacity protected is 700GB (100+500+100) but NetBackup eliminated the whitespace at source and hence just 330GB (60+250+20) needs to be processed. This processing efficiency is achieved even before deduplication engine kicks in! The 330GB of data is deduplicated and data blocks with unique fingerprints are shipped to deduplication storage. The deduplication savings depends on whether data with similar fingerprints are already in the deduplication pool or not.

Now let us induce changes to the virtual machine.

Imagine that the MS SQL Server is hosting both OLTP and Data mining workloads. Because of OLTP workloads, data files were changed by 10%. The SQL DBA also got a request to create a new data file of size 40GB to be used as a scratch space for data mining operations. The data-mining job finished and DBA was asked to delete the scratch space.

Now is the time to run backup again. 

VMware CBT had been logging changes to all VMDK files since the previous backup. All the changes since the previous backups are returned to backup application. For VMDK 2, the changes will be 10% of 250GB plus 40GB scratch file data mining load. That totals to 65GB of changes.

Because of Symantec V-Ray, something interesting happens here that is unique in the industry. VMware CBT reports the 40GB worth of changes from the scratch file and all backup vendors would religiously backup these blocks.  However those blocks won’t be recoverable as those belong to a deleted file with no inode associated with data blocks. That is wasteful cost in doing backups to read and move unnecessary data. I call this ‘CBT penalty’. If CBT was not in the picture, you won’t be wasting energy in moving unnecessary data. With Symantec V-Ray (from an earlier blog in this series, you may already know that Veritas Mapping Services, VxMS, is the secret sauce behind NetBackup’s implementation of Symantec V-Ray) NetBackup Accelerator for VMware has the ability to detect deleted blocks by evaluating the file system inode map during this phase and hence those 40GB of data is neither read nor shipped! Only the 25GB of useful changed blocks are read and injected into deduplication engine.

The deduplication engine deduplicates the changes blocks and if fingerprints are unique, the blocks are shipped to storage. The full backup image is created inline with references to fingerprints from changed blocks and previously stored blocks.

Now imagine a highly virtualized vSphere data center or vCloud environment where these types of operations are constantly taking place. VMware CBT helps significantly in detecting the changed blocks but that is just the first step in optimizing resources needed to perform backups. In combination with Symantec V-Ray and NetBackup Optimized Synthetics, now you are able to satisfy those workloads with high RPOs and RTOs without incurring CBT penalty.

Related blogs: Nuts and bolts in NetBackup for VMware: What is new in NetBackup 7.6?

Earlier blogs in Nuts and bolts series