[systemd-devel] How to stop systemd-udevd reading a device after dd

Akira Hayakawa ruby.wktk at gmail.com
Thu Oct 12 16:01:01 UTC 2017

I have a device /dev/sdb1 and let's trace the block request by blktrace

$ sudo blktrace -d /dev/sdb1

When I write 4KB using dd
$ sudo dd if=/dev/zero of=/dev/sdb1 oflag=direct bs=4k count=1

The block trace (after blkparsed) is write request as expected
  8,17   2        2     0.000003171  5930  Q  WS 2048 + 8 [dd]

followed by a unexpected read from systemd-udevd
  8,17   7        2     0.001755563  5931  Q   R 2048 + 8 [systemd-udevd]

My first question is what is: this read request?

And I want to stop the read request because it makes it difficult to test kernel code.
So the second question is: how can I stop the read request?
Actually I found this behavior when I test my module https://github.com/akiradeveloper/dm-writeboost
Since I need to create/remove devices during the tests, there is no choice to stop udev.service.

FYI, I am using Debian 9.2 

- Akira

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