vSphere changed block tracking: A powerful weapon for backup applications to shrink backup window

Changed block tracking is not a new technology. Those who have used Storage Foundation for Oracle would know that VERITAS file system (VxFS) provides no-data check points which can be used by backup applications to identify and backup just the changed blocks from the file systems where database files are housed.  This integration was in NetBackup since version 4.5 that was released 10 years ago! It is still used by Fortune 500 companies to protect mission critical Oracle databases that would otherwise require a large backup window with traditional RMAN streaming backups.

VMware introduced change block tracking (CBT) since vSphere 4.0 and is available for virtual machines version 7 or higher. NetBackup 7.0 added support for CBT right away. Backing up VMware vSphere environments got faster. When a VM has CBT turned on, it can track changes to virtual machine disk (VMDKs) sectors.  Its impact on VM performance is marginal. Backup applications with VADP (vStorage APIs for Data Protection) support can use an API (named QueryChangedDiskAreas) to identify and copy changed blocks from a particular point in time. This time point is identified using an argument named ChangeId in the API call.

VMware has made this quite easy for backup vendors to implement. Powerful weapons can be dangerous when not used with utmost care. An unfortunate problem in Avamar’s implementation of CBT came to light recently. I am not picking on Avamar developers here, it is not possible to predict all the edge cases during development and they are working hard to fix this data loss situation. As an engineer myself, I truly empathize with Avamar developers for getting themselves into this unfortunate situation. This blog is a humble attempt to explain what had happened as I got a few questions from the field seeking input on the use of CBT after the EMC reported issues in Avamar.

As we know, VADP lets you query the changed disk areas to get all the changes in a VMDK since a point in time corresponding to a previous snapshot. Once the changed blocks are identified, those blocks are transferred to the backup storage. The way the changed blocks are used by the backup application to create the recovery point (i.e. backup image) varies from vendor to vendor.

No matter how the recovery point is synthesized, the backup application must make sure that the changed blocks are accurately associated with the correct VMDK because a VM can have many disks. As you can imagine if the blocks were associated with the wrong disk in backup image; the image is not an accurate representation of source. The recovery from this backup image will fail or will result in corrupt data on source.

The correct way to identify VMDK is using their UUIDs which are always unique. Using positional identifies like controller-target-LUN at the VM level are not reliable as those numbers could change when some of the VMDK are removed or new ones are added to a VM. This is an example of disk re-order problem. This re-order can also happen for non-user initiated operations. In Avamar’s case, the problem was that the changed blocks belonging one VMDK was getting associated with a different VMDK in backup storage on account of VMDK re-ordering. Thus the resulting backup image (recovery point) generated did not represent the actual state of VMDK being protected.

To make the unfortunate matter worse, there was a cascading effect. It appears that Avamar’s implementation of generating a recovery point is to use the previous backup as the base. If disk re-order happened after nth backup, all backups after nth backup are affected on account of the cascading effect because new backups are inheriting the base from corrupted image.

This sounds scary. That is how I started getting questions on reliability of CBT for backups from the field. Symantec supports CBT in both Backup Exec and NetBackup. Are Symantec customers safe?

Yes, Symantec customers using NetBackup and Backup Exec are safe.

How do Symantec NetBackup and Backup Exec handle re-ordering? Block level tracking and associated risks were well thought out during the implementation. Implementation for block level tracking is not something new for Symantec engineering because such situations were accounted for in the design for implementing VxFS’s no-data check point block level tracking several years ago.

There are multiple layers of resiliency built-in Symantec’s implementation of CBT support. I shall share oversimplified explanations for two of those relevant in ensuring data integrity that are relevant here.

Using UUID to accurately associate ChangeId to correct VMDK: We already touched on this. UUID is always unique and using it to associate the previous point in time for VMDK is safe. Even when VMDKs get re-ordered in a VM, UUID stays the same. Thus both NetBackup and Backup Exec always associate the changed blocks to the correct VM disk.

Superior architecture that eliminates the ‘cascading-effect’:  Generating a corrupted recovery point is bad. What is worse is to use it as the base for newer recovery points. The corruption goes on and hurt the business if left unnoticed for long time. NetBackup and Backup Exec never directly inject changed blocks to an existing backup to create a new recovery point. The changed blocks are referenced separately in the backup storage. During a restore, NetBackup recreates the point in time during run-time. This is the reason NetBackup and Backup Exec are able to support block level incremental backups even to tape media! Thus a corrupted backup (should that ever happen) never ‘propagates’ corruption to future backups.

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