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I have some questions on cgroup blkio controller.
I want to limit the IOPS on a specific process. This process uses IO redirection to write the stdout contents into a file. I tried to put the process into a cgroup. However, it seems the cgroup didn't catch the IO operations on the process, thus not able to limit the IOPS.

I did a small experiment to simulate the process. Here's what I did:

cgexec -g blkio:/dummy_group --sticky dd bs=1 if=/dev/zero of=/dev/stdout count=30000000 > /tmp/test

I planed to generate a 30MB file using dd, output it to the stdout, and then redirect to the file. I expected the write operations should be recorded in the cgroup statistics. However, when I read the "/cgroup/blkio/dummy_group/blkio.throttle.io_serviced" file, the 'Write' number didn't change !

Then I did the similar tasks using cp command, the 'Write' number really changed:

cgexec -g blkio:/dummy_group --sticky cp /tmp/test /tmp/test2

I'm wondering if I've done something wrong to achieve this goal. What's the correct method to monitor and limit processes whose IO is performed using IO redirection?

Thanks.

  • For a closer comparison, use the first command but with of=/tmp/test. – JdeBP Jun 17 '16 at 8:28

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