Avamar Image backup: Backed Disk size

The logical size of the backup will always be the full size of the disks (in this case 100GB). Only the in-use portion of the disk will use physical capacity on the back-end since the rest of the disk is a long series of 0s. Less than 20GB of back-end capacity will be used, with the exact amount depending on how well those 20GB of data compress and de-duplicate.

A little more detail:

In the worst case, Avamar will back up all 100GB. This will only happen if the VMware call that provides Avamar with information about what disk extents are in use fails. The 80GB of 0s at the end of the disk will be chunked up into identical chunks of about 44KB, then compressed. These identical chunks will be de-duplicated away and consume almost no space on the back-end.

For L0 backups and incremental backups where Change Block Tracking (CBT) is disabled (or retrieving the changed block list fails), Avamar will back up the 20GB that are in use. These 20GB will be chunked, compressed and de-duplicated as normal. Data already present on the server will not be transferred or stored again.

For incremental backups with CBT enabled, Avamar will request a list of the changed blocks from VMware. For each changed block, Avamar will merge the block with a copy of the surrounding blocks, then re-chunk, compress and de-duplicate that section of the data. This "retrieve and merge" behaviour is the reason that VMs with high daily change rates sometimes perform better with CBT disabled.

Avamar always backs up logical disks completely, regardless of whether they are thick or thin. For thin disks, the in-use extents are recorded during the backup so the disk can be restored as a thin disk later.

I hope that helps. Please let me know if you have any questions.