Erik Darling notes that databases using Transparent Data Encryption now support backup compression:
First, the database without a Max Transfer Size at the bottom was a full backup I took with compression, before applying TDE. It took a little longer because I actually backed it up to disk. All of the looped backups I took after TDE was enabled, and Max Transfer Size was set, were backed up to NUL. This was going to take long enough to process without backing up to Hyper-V VM disks and blah blah blah.
The second backup up, just like the blog man said, no compression happens when you specify 65536 as the Max Transfer Size.
You can see pretty well that the difference between compressed backup sizes with and without TDE is negligible.
Check it out, including the table Erik put together. I’m glad that backup compression is now supported, although I’m kind of curious how they can do that while retaining encrypted backups—are they decrypting data, writing to backup (and compressing), and then encrypting the backup? That’d be worth checking out with a hex editor.