Vmware CBT (change block tracking) : ctk files and multiple backup systems ?

[Sharantyr3]

Hi,

We have 2 datarecovery that runs backups of the SAME virtual machines but on different day.

Later, we will have a dedicated backup solution, so we will use only 1 VDR + this external backup system.

The external backup system use CBT too.

Now my question is : how CBT does when multiple backup systems ask for "what's the latest changed blocks since the last backup?"

I mean if I run, on same VMs, VDR1 saturday, and VDR2 sunday, will VDR2 always only have the changed blocks from saturday to sunday, and misses all the week's changed blocks ????!!!!

Anyone could provide me a technical in depth explanation of CBT mechanism ?

Thanks

[a_p_]

Please take a look at http://www.vmware.com/support/developer/vddk/vcb_vsphere_backup.pdf (Changed Block Tracking on Virtual Disks - page 21). This explains the creation and use of ChangeID's used to track/query changed blocks.

André

[Sharantyr3]

It's still unclear to me. The mentioned document states

If you have a saved ChangeId, it identifies the last time you performed a backup, and tells the changed block tracking
logic to identify those that have occurred since the time indicated by the ChangeId

But reading this, I assume the process can be seen this way :

bckp-app : Hi, I need to make a incremental backup

ctk : What's your recorded Changeid from your latest backup ?

bckp-app : Changeid

ctk : Ok here is the list of changed blocks since your last Changeid, and your new Changeid may be ...

Right ?

But then, my question would be : how long a ctk file can keep track of ChangeID and provide a correct list of blocks changed since this ChangeID ?

[a_p_]

how long a ctk file can keep track of ChangeID and provide a correct list of blocks changed since this ChangeID ?

It doesn't need to. Let's assume (to keep it simple) the current changeID is 100 and a backup application asks for changes made since 75, it will get a list of all blocks with a changeID >= 75.

André

[Sharantyr3]

Ok so I think I misunderstood the CBT concept.

What you're saying is the ctk file contains the full map of all blocks of the vmdk, each block associated with a ChangeID ?

[a_p_ · Accepted solution]

Yes, once a block is modified the corresponding location in the mapping file is tagged with the new changeID.

André

[Sharantyr3]

Thanks for the answers