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iotest 129

From: Max Reitz
Subject: iotest 129
Date: Tue, 12 Jan 2021 18:44:46 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.0


tl;dr: I have some troubles debugging what’s wrong with iotest 129. It wants to check that 'stop' does not drain a block job, but to me it seems like that’s exactly what’s happening with the mirror job.

For quite some time, I’ve had 129 disabled in my test branch because it fails all the time for me. Now it came up again in Vladimir’s async backup series, and so I tried my hands at debugging it once again.

Recap: 129 writes 128M to some image, then runs a block job from there (while the source drive is supposedly throttled), then issues a stop command, and checks that the job is not drained. I.e., still running.

(It checks the “running” state via @busy, which is probably wrong; it should verify that @state == 'running' (which wasn’t available back in 2015), but that’s not really why I’m writing this mail.)

Like the last time I tried
(https://lists.nongnu.org/archive/html/qemu-block/2019-06/msg00499.html) I can see that block jobs completely ignore BB throttling: If you apply the attachment show-progress.patch, you’ll probably see some progress made while the test waits for five seconds. (Here, on tmpfs, mirror and commit get to READY, and backup completes.)

OK, so now that block jobs don’t completely break with filters, you can instead use a filter node on the source (as I tried in the patch referenced above). And to fully fix the test, we’d also replace the @busy == True check by @status == 'running'. That’s the attachment filter-node-show-progress.patch.

If I apply that, I can see that now there actually is some throttling. After the time.sleep(5), mirror and commit get to exactly 1 MB, and backup gets to 1.0625 MB. Good.

However, after the stop is issued, backup stays at 1.2 MB (good), but mirror and commit progress obviously without throttling to 30, 40, 50 MB, whatever. So it appears to me they are drained by the stop. I.e., precisely what the iotest is trying to prove not to happen.

I plan to continue investigating, but I just wanted to send this mail to see whether perhaps someone has an idea on what’s going on.

(My main problem is that bisecting this is hard. AFAIK the throttling applied in master:129 has been broken ever since blockdev throttling was moved to the BB. Even without the “Deal with filters” series, you can use at least mirror sync=full from filter nodes, so I tried to use filter-node-show-progress.patch for just test_drive_mirror(), but that only works back until fe4f0614ef9e361d (or rather 0f12264e7a4145 if you prefer not to get a SIGABRT). Before that commit, it hangs.)


Attachment: show-progress.patch
Description: Text Data

Attachment: filter-node-show-progress.patch
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