On a recent webinar, I was asked about the recovery mechanism in vRanger Pro 4.5. Specifically, I was asked: "Does vRanger Pro merge all backup copies into a single image in the repository?"
Translation: This was a question about Synthetic Full backup. So, of course, I said: "No - and we have good reasons that we don't do that, including keeping backup images intact with no risk to their recoverability." I have previously blogged about the problems with image-based backup products that base their design on Synthetic Full, so I'm fully on the record on this point. As is Jason Mattox, as he stated even more pointedly in his criticism.
However, our webinar participant was not done. He followed up his initial question with the following: "But, doesn't the lack of a single, merged backup image mean then that the recovery process is inefficient? Don't you have to go through multiple passes for recovery of the same blocks over and over, like traditional backup products?"
I was very glad that this participant asked the question. He revealed confusion about a topic about which many people likely have questions. The topic is Synthetic Restore.
What is Synthetic Restore?
Synthetic Restore is a feature that has been in the vRanger Pro product for a while now. It's purpose is to avoid the exact problem about which the particpant asked: rather than having to restore multiple copies of the same block from multiple point-in-time copies, Synthetic Restore speeds recovery by reading only the most current PIT copy of each block - and reading it only one time.
The picture illustrates the point. vRanger Pro interrogates the incremental and differential backup images first, and pulls out their blocks. It does this in reverse order of when the backup copies were made, starting with the PIT which is closest to the recovery. Any block which is present in the closest backup image, is not read again from previous backup images.
The Synthetic Restore process looks very similar to Synthetic Full backup. However, the difference is very large in that Synth Full merges all blocks together ON WRITE into the backup repository - which risks the integrity of the backup image every time a new backup job is completed.
In contrast, Synthetic Restore leaves the backup images intact and untouched after they have been written into the repository. Even if a subsequent backup job is interrupted during the write process, previous backup images are still present - untouched and uncorrupted.
In this way, vRanger Pro delivers the speed of recovery benefit that you need while avoiding the risk to the integrity of images in the backup repository.