Thin-provisioned virtual disk is created on first write. So the first-time write to thin-provisioned disk involves extra overhead compared to thick disk, whether using NBDSSL or HotAdd. This is due to block allocation overhead, not VDDK advanced transports. However once thin disk has been created, performance is similar to thick disk, as discussed in the Performance Study of VMware vStorage Thin Provisioning.
When applications perform random I/O or write to previously unallocated areas of thin-provisioned disk, subsequent backups can be larger than expected, even with CBT enabled. In some cases, disk defragmentation might help reduce the size of backups.
