Ahhhh that makes sense. Unless you have a huge number of backup files, I wouldn't think the overhead would be that big, and really on a multi-TB volume what's a few 60k here and there
True, but remember, the alignment is per-block within the backup file, not per-file, because blocks within the backup file are the elements we want to be able to clone, and clone operations have to be on cluster boundaries.
After compression, each block backed up by Veeam will have somewhere between 0-4K of overhead when stored on ReFS with 4K clusters, and somewhere between 0-64K of overhead when stored on ReFS volume with 64K clusters. Perhaps a simpler way to look at it, 4K clusters will have an average 2K overhead, and 64K cluster will have an average 32K overhead. That's 30K more overhead per source block, because the number of blocks in a given VM is fixed, based on the configured block size.
Simple example: Single VM with 10GB of data
10GB/1MB blocks = 10,000 blocks
10,000 blocks * 2K = 20MB estimated overhead with 4K clusters
10,000 blocks * 32K = 320MB estimated overhead with 64K clusters
Now let's calculate the overhead as a % of total space. Let's assume a somewhat typical 2.5x compression ratio so that means:
10GB/2.5x compression = 4GB VBK
So the backup file with no overhead would be 4GB so the overhead of 4K clusters (20MB) is 0.5% while the overhead with 64K clusters (320MB) is more like 8%. This means the resulting backup files are 0.5% and 8% larger than if it had no alignment, or roughly 7.5% difference between 4K and 64K.
Of course the math changes if the compression ratio changes, say we only get 1.5x compression, so suddenly the backup file is 6.65GB but the overhead per block doesn't change. Since the backup itself is larger, the fixed space of the overhead represents a smaller percentage, only ~5% with 64K clusters, and 0.3% with 4K.
Also, the math changes if you change the block size. If you went from the default of Local target (1MB blocks), to LAN target (512K blocks), suddenly that's 20,000 blocks and, since the alignment overhead is per-block, that doubles my overhead with 64K clusters to 640MB. The backup size is likely to be the same, so that 8% overhead just became 16%!
Once again, some of that overhead is made up for my having smaller allocation tables and I believe the proper balance is default settings for Veeam block size and 64K clusters, but it's interesting to understand the impact of it all.