December 9, 2010

how to fill your disk with random data

When using full-disk encryption, it is useful to prefill the disk in question with (pseudo)random data. This makes it harder to tell how much of the encrypted volume's space is already written to - in other words, how much data you have on the volume.

There are many ways to do it - specialized tools, reading from /dev/urandom (reasonably fast), reading from /dev/random (true randomness, but unless you have a HWRNG, it will take 1000 years to fill a disk). Trouble is, generating pseudorandom data is slow. While your average HDD can write at speeds over 50MB/s, you can only generate randomness at, say, 8MB/s (with one core, that is)

The usual recommended method is this:
dd if=/dev/urandom of=/dev/sda
It will take a very long time, because generating the random numbers is slower than writing them to the disk. The problem is that the kernel is only using one CPU core to generate the /dev/urandom stream - the CPU core on which your process runs.

Now if only there was some kind of a trick to make kernel use all four of my CPU cores...
You could, of course, run four dds and make them write to different areas of your disk - but wait, wouldn't that force the disk to seek back and forth? Wouldn't that be a little stupid? Yeah, I thought so.

That's why i wrote this tiny program called urandread. It will open four (or how many you need) processes to read from /dev/urandom, and then combine their output into a stream that is four times faster.
Then you can do this:
./urandread | dd of=/dev/sda
and you're BLAZING!



Anonymous said...

If /dev/urandom can't do 50MBps on single core perhaps it needs to be fixed.

Anonymous said...

Any chance you could repost your program? It's exactly what I was looking for, but the link is broken.