I was reading Eric Siebert’s excellent article on Changed Block Tracking(CBT) and the article on Punching Cloud on this new feature which is part of vSphere. CBT enables incremental backups of full VMDKs. Something that isn’t covered is what the “block” part of Changed Block Tracking actually stands for.
Someone asked me on the VMTN Communities and it’s something I had not looked into yet. The question was around VMFS block sizes and the way it could potentially have its effect on the size of a backup which uses CBT. The assumption was made that CBT on a 1MB block size VMFS volume uses 1MB blocks and on an 8MB block size VMFS volume uses 8MB blocks. This is not the case.
So what’s the size of the block that CBT refers to? Good question I’ve asked around and the answer is that it’s not a specific size but it has a variable size. The block always starts with 64KB and the bigger the VMDK becomes the bigger the blocks become.
Just for the sake of it:
- CBT is on a per VMDK level and not on a VMFS level.
- CBT has variable block sizes which are dictated by the size of the VMDK.
- CBT is a feature that lives within the VMKernel and not within VMFS.
- CBT is a FS Filter as shown in the VMworld slide below