The effect of file deletions on backup sizes

[pshute]

If I have a VM with a 1TB drive that's full, and I delete half the files, what effect will that have on backup sizes if I have "Exclude deleted file blocks" and CBT enabled?

Assuming the files I deleted were located randomly across the drive, many blocks will be affected, and the free space won't be contiguous. What effect will it have on the next incremental, and on later full backups?

What effect would defragmentation have?

[DavoudTeimouri]

I guess, there is no effect until next full backup. Incremental just move changes to backup file.

[pshute]

I think it did have an effect on the incrementals.

Recent figures for the data read from that disk have been around 2GB each day (it's not in use). One day I deleted about 127GB and defragmented, and the next incremental read 76GB and transferred 64GB for that VM. Next day I deleted another 190GB and defragmented, and the next incremental read 194GB and transferred 174GB for that VM.

It would have been interesting to see what those figures were if I hadn't defragmented. If there were blocks changed all over the disk due to the deletions, but little contiguous free space, perhaps the incrementals would have been even bigger. Or maybe not, perhaps the defragmenting changed more blocks that affect incremental size.

I'll delete more next week, and not defragment. If deleted file blocks are excluded, defragmenting should be unhelpful.

[skrause]

Unless you have massive fragmentation causing performance issues or you are trying to shrink the size of the volume, it is commonly considered best practice to avoid defragmentation operations on windows VMs. This is especially the case when you are using thin provisioned disks.

Any time the location of the files on disk is modified (which defragmentation, deduplication, or stuff like "s-delete" does) then Veeam will flag that block as changed.

[foggy]

Correct, defragmentation results in massive block changes, so it is recommended to be performed prior the active full, to eliminate the effect.