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Ubuntu – Why are /dev/random and /dev/urandom stopping after 32MiB?

I get this output on very single commandline listed below:

0+1 records in
0+1 records out
33554431 bytes (34 MB, 32 MiB) copied, 0.14532 s, 231 MB/s

Here are the different options I tried:

a) dd if=/dev/random of=/dev/null status=progress bs=1G count=1

b) dd if=/dev/urandom of=/dev/null status=progress bs=1G count=1

c) dd if=/dev/urandom of=/dev/null status=progress bs=100M count=1

d) dd if=/dev/urandom of=/dev/null status=progress bs=50M count=1

e) dd if=/dev/random of=/dev/null status=progress bs=50M count=1

f) dd if=/dev/random of=/dev/null status=progress bs=33M count=1

g) dd if=/dev/urandom of=/dev/null status=progress bs=33M count=1

This dd if=/dev/urandom of=/dev/null status=progress bs=10M count=1 is doing as expected and outputs:

1+0 records in
1+0 records out
10485760 bytes (10 MB, 10 MiB) copied, 0.0518184 s, 202 MB/s

I tried this with MX Linux (Kernel 5.10) and with KDE neon and with Ubuntu 20.04

Does someone have a under-the-hod answer? Or some possible ideas?

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2

Answers


  1. 0

    From the Linux man pages:

    Since Linux 3.16, a read(2) from /dev/urandom will return at most
    32 MB

    The reason has to do with the fact that it’s a special character file interfacing with an underlying driver rather than a "regular" file. The underlying driver clamps the output size to 32 MB in this case. Another answer here provides more details on this matter.

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  2. 0

    bs, the buffer size, means the size of a single read() call done by dd.

    (For example, both bs=1M count=1 and bs=1k count=1k will result in a 1 MiB file, but the first version will do it in a single step, while the second will do it in 1024 small chunks.)

    I came with solution that reads 32 chunks 32 MB large which makes 1GB:

    dd if=/dev/urandom of=output bs=32M count=32
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