VMware

Is There Such A Thing As A “Delta” or “Incremental” Snapshot in vSphere?

Hello – We are deploying a new backup system (Rubrik) and I’m being told that it will be faster for our VMware environment since it does an initial full backup and then everything afterwards are incremental/delta backups. What I’d like to know is if there is such a thing as a “delta” or “incremental” snapshot in VMware? My understanding is that, even if the backup is an incremental, the snapshot that the backup tool is initiating with VMware is of the full disk. In fact, I don’t know how VMware itself would even know what the “delta” information in the disk would be at the time the snapshot is initiated. So my assumption is that VMware will do the same thing when it creates the snapshot regardless of whether the backup tool is executing a full or incremental job. Furthermore, I assume that only the backup tool would be able to scan the snapshot (disk) to look for change data that it is interested in. Can anyone offer some insight on this? Thanks!!


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2 Comments

  1. So Rubrik, and most other software vendors are using a few VMware technologies to make this all happen.

    1. VADP- These are the APIs for Data Protection software, aka, Rubrik, Veeam, CommVault, etc.

    2. Snapshots- Most backup software takes a snapshot, but the reason it does this isn’t the reason you’re thinking- the snapshot exists so the backup is of a point in time- the snap happens, and the base VMDK is now frozen, as it was when the snapshot is taken, this lets the backup software back it up without worrying about incoming changes. This is kind of equivalent to Microsoft VSS, but it doesn’t break all the time.

    3. CBT- Changed Block Tracking. This is a really cool vmware technology that lets backup software ask the hypervisor “what’s new” and focus on that. A file with -ctk is created and it tracks which blocks have received changes or writes since the last time you checked in. Backup software focuses on those, and ta-da! incremental backups always. If CBT returns invalid data though, all your backups are worthless, so CBT bugs are super serious and if you see those in release notes they need to be patched immediately. Also a good reason to have periodic test restores to make sure everything’s good.

  2. Hm, first I guess you need to understand that… Snapshots aren´t backups, but can used during the process! 😀

    VMware use CBT (Change Block Tracking) for incremental backups, and it can analyse what changed at block level. So it´s smart and fast, and does not has the “overhead” that a snapshot cause (mainly disk utilization).

    In a simple way, imagine that a flag is set on every block that is changed after the last backup.

    I don´t know Rubrik, but every backup solution since 4.x IIRC use CBT.

    If you open your vcenter/esxi during the backup, you´ll see that a snapshot will be taken, but the backup tool won´t use it (the snapshot) for the backup itself, it´s only used to “free” the VMDK and the CBT magic will happen.

    Sorry about my English, but tried to superficially explain how the VMware´s backup works.

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