[w8rbt]

Zeros are far more efficient and much faster to generate.

Not sure why some have downvoted me. It's easy to verify.

    time dd if=/dev/zero of=file1.bin bs=1M count=600

    600+0 records in
    600+0 records out
    629145600 bytes (629 MB) copied, 1.57948 s, 398 MB/s

    real	0m1.651s
    user	0m0.004s
    sys	0m0.688s

    time dd if=/dev/urandom of=file2.bin bs=1M count=600

    600+0 records in
    600+0 records out
    629145600 bytes (629 MB) copied, 37.7828 s, 16.7 MB/s

    real	0m37.786s
    user	0m0.004s
    sys	0m37.784s

[pstrateman]

Linux kernels after 4.8 use chacha which is significantly faster than the previous implementation. (~250MB/s on my laptop).

[sliken]

I didn't downvote, but /dev/urandom isn't a particularly large bottleneck for any recent machine. I've got a 4 generation old I5 (3570) dd of /dev/urandom gets 300MB/sec for me. SATA is only capable of twice that, so it's not a particularly large bottleneck.

I do wonder what machine you used that's 20 times slower than a 4 generation old i5.

[rini17]

I had similar problem with urandom, instead used openssl like this and it's fast enough:

openssl enc -aes-256-cbc -in /dev/zero -out /dev/sdX

[sliken]

Interesting. Looks like for newer kernels (newer than 4.8) that's about twice as fast. But for older kernels it's around 20 times as fast.

[gist]

Thanks for this. I actually spent a bit of time playing around and learning about dd based on the above. I found that on my machine it is also slower as well. Never really had a need to use dd even with many years of playing with Unix.

This post appears to have the reason:

https://superuser.com/questions/359599/why-is-my-dev-random-...

(This raises the question of why using openssl (per other comment) makes it faster.)