[Qemu-devel] [Bug 1689499] Re: copy-storage-all/inc does not easily converge with load going on

[ChristianEhrhardt]

Ok, I found an hour to set up a test environment.
I already had all the bisect script written until the systems were ready, but 
unfortunately it is not reproducible enough with my build from git as of 
today's master - out of 8 tries it showed as less of a slowdown once, and never 
to the hours of wait that I saw when testing on a wider scope.

If my regular testing triggers these issues again I'll try to sort out
what the difference is between this and my bisect environment (I set the
latter up with the scripts that do the former), but until then I'm kind
of out of options for now :-/.


** Changed in: qemu
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/1689499

Title:
  copy-storage-all/inc does not easily converge with load going on

Status in QEMU:
  Incomplete

Bug description:
  Hi,
  for now this is more a report to discuss than a "bug", but I wanted to be 
sure if there are things I might overlook.

  I'm regularly testing the qemu's we have in Ubuntu which currently are
  2.0, 2.5, 2.6.1, 2.8 plus a bunch of patches. And for all sorts of
  verification upstream every now and then.

  I recently realized that the migration options around 
--copy-storage-[all/inc] seem to have got worse at converging on migration. 
Although it is not a hard commit that is to be found, it just seems more likely 
to occur the newer the qemu versions is. I assume that is partially due to 
guest performance optimization that keep it busy.
  To a user it appears as a hanging migration being locked up.

  But let me outline what actually happens:
  - Setup without shared storage
  - Migration using --copy-storage-all/--copy-storage-inc
  - Working fine with idle guests
  - If the guests is busy the migration does take like forever (1 vCPU that are 
busy with 1 CPU, 1 memory and one disk hogging processes)
  - statistically seems to trigger more likely on newer qemu's (might be a red 
herring)

  The background workloads are most trivial burners:
  - cpu: md5sum /dev/urandom
  - memory: stress-ng -m 1 --vm-keep --vm-bytes 256M
  - disk: while /bin/true; do dd if=/dev/urandom of=/var/tmp/mjb.1 bs=4M 
count=100; done

  We are talking about ~1-2 minutes on qemu 2.5 (4 tries x 3
  architectures) and 2-10+ hours on >=qemu 2.6.1.

  I say it is likely not a bug, but more a discussion as I can easily avoid 
hanging via either:
  - timeouts (--timeout, ...) to abort or suspend to migrate it
  - --auto-converge ( I had only one try, but it seemed to help by slowing down 
the load generators)

  So you might say "that is all as it should be, and the users can use
  the further options to mitigate" and I'm all fine with that. In that
  case the bug still serves as a "searchable" document of some kind for
  others triggering the same case. But if anything comes to your mind
  that need better handling around this case lets start to discuss more
  deeply about it.

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