Is it smart to use NTFS compression on a bitlocker encrypted disk?

Is it smart to use NTFS compression on a bitlocker encrypted disk?

My intuition says that this should serve no benefit. However I just clean installed windows 10 pro and all of my non-system files are compressed. I'm getting almost 50% compression on my ssd. 
If they are encrypted, how could this be possible?
Am I correct in thinking there shouldn't be any benefit, eventhough it looks like there is?


Answer 1:

I think I understand your logic, but the order of things is important…

After encryption encrypted files look like if they are containing random data. And random data doesn’t compress well.
So storing already encrypted files in a compressed-archive format (like ZIP, RAR or 7Z) doesn’t yield much benefit in terms of compression ratio. Storage space may actually increase instead of decrease.

But here you have things the other way around:
Files are compressed first (at the NTFS level, before actually saving them) to make them smaller.
Then these compressed files are stored on a disk with full-disk encryption.
The encryption process doesn’t know (or care) what it is encrypting. To BitLocker, which operates at the block-level below the filesystem, it is all just blocks of data and the content is irrelevant. The encrypted block while be exactly the same size as the original un-encrypted block, just the content will be scrambled.
So you get the space-saving of the compression and the encryption and they don’t affect each other.

Answer 2:

If your dataset will benefit from compression then using NTFS compression will still provide that benefit, because of the order data is written in.

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If you think about this as acting at different layers, it may be clear why.

Data to Be Saved -> NTFS Compressed Filesystem -> Bitlocker Encryption -> Disk

Because (when writing) the NTFS Compression is taking place before Bitlocker encryption, the files will indeed be smaller.

To clarify, although encryption may look pretty much random on a raw disk, by the time it is written on, Bitlocker is decrypted it, so the data does not look random to the OS, and thus it can benefit from encryption.

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