Use /dev/zero for disk rate check

[q-wertz]

Pull Request Details

/dev/random is slow by design and probably a bottleneck in the check.
/dev/zero should be faster.

Description

On test we triggered the warning of a too slow disk write speed.
The test internally uses dd to test the disk write speed but with the
if=/dev/random which will be slow as soon as random source is
depleted. See e.g.

~ $ dd if=/dev/zero of=/home/username/Downloads/test.dat bs=8k count=10k
10240+0 records in
10240+0 records out
83886080 bytes (84 MB, 80 MiB) copied, 0,0512609 s, 1,6 GB/s
~ $ dd if=/dev/random of=/home/username/Downloads/test.dat bs=8k count=10k
10240+0 records in
10240+0 records out
83886080 bytes (84 MB, 80 MiB) copied, 0,292775 s, 287 MB/s

This leads to inaccurate test results and warnings that might unsettle
the user.

Related Issue

Which devices/areas does this affect?

All devices.

Testing Done

See above.

Checklist

• I have read the CONTRIBUTING document.
• My code follows the code style of this project. See CODING.md.
• I have updated the documentation accordingly.
• I have added tests to cover my changes, and all previous tests pass.
• I have checked all compat numbers if they need updating (FPGA compat,
  MPM compat, noc_shell, specific RFNoC block, ...)