3 ms·

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=1

by w8rbt 9y ago

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

sliken 9y ago
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 9y ago
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 9y ago
  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 9y ago
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-so-slow-when-using-dd https://superuser.com/questions/359599/why-is-my-dev-random-... (This raises the question of why using openssl (per other comment) makes it faster.)
pstrateman 9y ago
Linux kernels after 4.8 use chacha which is significantly faster than the previous implementation. (~250MB/s on my laptop).

by w8rbt 9y ago

sliken 9y ago

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 9y ago

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 9y ago
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 9y ago

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-so-slow-when-using-dd https://superuser.com/questions/359599/why-is-my-dev-random-... (This raises the question of why using openssl (per other comment) makes it faster.)

pstrateman 9y ago

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