[Bug 1895487] Re: 'qemu-img convert' performance regression when target is a LVM volume

[Chris MacNaughton]

** Also affects: cloud-archive/stein
   Importance: Undecided
       Status: New

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Title:
  'qemu-img convert' performance regression when target is a LVM volume

Status in Ubuntu Cloud Archive:
  New
Status in Ubuntu Cloud Archive stein series:
  New
Status in qemu package in Ubuntu:
  Won't Fix

Bug description:
  Running Ubuntu Server 18.04 with the Ubuntu Cloud Archive repository
  on release 'bionic-updates/stein'

  qemu-utils version 1:3.1+dfsg-2ubuntu3.7~cloud0

  With bionic-updates/rocky and (stock bionic-updates) qemu-utils
  version 1:2.11+dfsg-1ubuntu7.31, the following command takes ~35
  seconds to complete:

  # qemu-img convert -p -O raw -t none -f qcow2 some-qcow2.img
  /dev/mapper/cinder--volumes-volume--9d18daa7--691c--44bc--b4c8--
  a60cc70a435d

  With bionic-updates/stein and qemu-utils version 1:3.1+dfsg-
  2ubuntu3.7~cloud0, the same command does not progress visibly (-p
  displays a rough percentage done), still displaying 0% finished after
  two minutes.

  Basic package level bisecting shows that qemu-utils version 1:4.0
  +dfsg-0ubuntu9.8~cloud0 (from bionic-updates/train) fixes the
  regression, behaving like 1:2.11+dfsg-1ubuntu7.31 again.

  After verifying the bug also exists upstream, I bisected with the
  following results:

  - The bug was introduced with commit
  34fa110e424e9a6a9b7e0274c3d4bfee766eb7ed (between v3.0.0-rc2 and rc3)

  - The bug was fixed with commit
  c9fdcf202f19fc2acdcb1ee0522ff5d61bf8c906 (between v3.1 and v4.0),
  which is part of this patchset:
  https://patchwork.kernel.org/cover/10865751/

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