I stumbled across this trick elsewhere several years ago, but I don't remember where. I needed to generate 10TiB of hard-to-compress data, and /dev/urandom is somewhat slow (400-ish MB/s)
Most major CPUs over the past several years have acceleration for AES-256 ciphers. So one way to produce lots of random-ish, hard to compress data is to leverage openssl.
For OpenSSL 1.1 onwards, using current epoch as the seed:
$ openssl enc -aes-256-ctr -pbkdf2 -pass pass:"$(date '+%s')" < /dev/zero
Note you could replace that call out to date with a straight password instead, but you'll always get the same data each time out the other side:
$ openssl enc -aes-256-ctr -pbkdf2 -pass pass:"pass" < /dev/zero
According to my notes the original version used to feed in some data from /dev/urandom piped through base64 as the seed, but that feelse like overkill to me:
$ openssl enc -aes-256-crt -pbkdf2 -pass pass:"$(dd if=/dev/urandom bs=128 count=1 2>/dev/null | base64)" < /dev/zero
The original version used to use the -nosalt argument, I'm not sure that provides value either.
Note: On older versions of OpenSSL you may need to drop the -pbkdf2 argument to get it to work.