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[Eli Qiao <liyong.qiao@xxxxxxxxx>]

This is already fixed by this commit
I2c3df909a783e1480d3ab4ca10b34b84ac9e4b5f

** Changed in: nova
       Status: In Progress => Won't Fix

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https://bugs.launchpad.net/bugs/1484992

Title:
  Converting image from qcow2 to raw is slow for images with large
  virtual size

Status in OpenStack Compute (nova):
  Won't Fix

Bug description:
  In this thread:

  http://lists.openstack.org/pipermail/openstack/2015-August/013614.html

  [qote]
  Running cmd (subprocess): sudo nova-rootwrap /etc/nova/rootwrap.conf qemu-img convert -O raw /var/lib/nova/instances/_base/999f7fff2521e4a7243c9e1d21599fd64a19b42e /dev/openstack-controller01-ky01-vg/5f831046-435c-4636-8b71-a662327b608c_disk

  Well, I understand what this step is doing - it's copying raw image to
  lvm.

  How can we speed it up?
  I don't wanna have instance from 100GB image booted for 4 hours
  [/quote]

  Regardless of how much data is in the source file, qemu-img has to
  write out the full virtual size on the target block dev to ensure any
  pre-existing data is overwritten. THis may involve writing 100's of GB
  of zeros for large images.

  qemu-img does has an optimization to let it use 'discard' as a fast
  alternative to writing zeros, but this requires that qemu-img has disk
  caching disabled , eg  -t none. We should investigate to see if this
  improves performance for converting large images to block devices, and
  if so, enable this flag.

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