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[Qemu-devel] [Bug 1538541] [NEW] qcow2 rejects request to use preallocat


From: Daniel Berrange
Subject: [Qemu-devel] [Bug 1538541] [NEW] qcow2 rejects request to use preallocation with backing file
Date: Wed, 27 Jan 2016 12:55:58 -0000

Public bug reported:

The 'preallocation=full' option to qemu-img / qcow2 block driver
instructs QEMU to fully allocate the host file to the maximum size
needed by the logical disk size.

$ qemu-img create -f qcow2 -o preallocation=full base.qcow2 200M
Formatting 'base.qcow2', fmt=qcow2 size=209715200 encryption=off 
cluster_size=65536 preallocation='full' lazy_refcounts=off refcount_bits=16

$ ls -alhs base.qcow2 
201M -rw-r--r--. 1 berrange berrange 201M Jan 27 12:49 base.qcow2


When specifying a backing file for the qcow2 file, however, it rejects the 
preallocation request

$ qemu-img create -f qcow2 -o preallocation=full,backing_file=base.qcow2 
front.qcow2 200M
Formatting 'front.qcow2', fmt=qcow2 size=209715200 backing_file='base.qcow2' 
encryption=off cluster_size=65536 preallocation='full' lazy_refcounts=off 
refcount_bits=16
qemu-img: front.qcow2: Backing file and preallocation cannot be used at the 
same time


It might seem like requesting full preallocation is redundant because most data 
associated with the image will be present in the backing file, as so the top 
layer is unlikely to ever need the full preallocation.  Rejecting this, 
however, means it is not (officially) possible to reserve disk space for the 
top layer to guarantee that future copy-on-writes will never get ENOSPC.

OpenStack in particular uses backing files with all images, in order to
avoid the I/O overhead of copying the backing file contents into the
per-VM disk image. It, however, still wants to have a guarantee that the
per-VM image will never hit an ENOSPC scenario.

Currently it has to hack around QEMU's refusal to allow backing_file +
preallocation, by calling 'fallocate' on the qcow2 file after it has
been created. This is an inexact fix though, because it doesn't take
account of fact that qcow2 metadata can takes some MBs of space.

Thus, it would like to see preallocation=full supported in combination
with backing files.

** Affects: qemu
     Importance: Undecided
         Status: New

-- 
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https://bugs.launchpad.net/bugs/1538541

Title:
  qcow2 rejects request to use preallocation with backing file

Status in QEMU:
  New

Bug description:
  The 'preallocation=full' option to qemu-img / qcow2 block driver
  instructs QEMU to fully allocate the host file to the maximum size
  needed by the logical disk size.

  $ qemu-img create -f qcow2 -o preallocation=full base.qcow2 200M
  Formatting 'base.qcow2', fmt=qcow2 size=209715200 encryption=off 
cluster_size=65536 preallocation='full' lazy_refcounts=off refcount_bits=16

  $ ls -alhs base.qcow2 
  201M -rw-r--r--. 1 berrange berrange 201M Jan 27 12:49 base.qcow2

  
  When specifying a backing file for the qcow2 file, however, it rejects the 
preallocation request

  $ qemu-img create -f qcow2 -o preallocation=full,backing_file=base.qcow2 
front.qcow2 200M
  Formatting 'front.qcow2', fmt=qcow2 size=209715200 backing_file='base.qcow2' 
encryption=off cluster_size=65536 preallocation='full' lazy_refcounts=off 
refcount_bits=16
  qemu-img: front.qcow2: Backing file and preallocation cannot be used at the 
same time

  
  It might seem like requesting full preallocation is redundant because most 
data associated with the image will be present in the backing file, as so the 
top layer is unlikely to ever need the full preallocation.  Rejecting this, 
however, means it is not (officially) possible to reserve disk space for the 
top layer to guarantee that future copy-on-writes will never get ENOSPC.

  OpenStack in particular uses backing files with all images, in order
  to avoid the I/O overhead of copying the backing file contents into
  the per-VM disk image. It, however, still wants to have a guarantee
  that the per-VM image will never hit an ENOSPC scenario.

  Currently it has to hack around QEMU's refusal to allow backing_file +
  preallocation, by calling 'fallocate' on the qcow2 file after it has
  been created. This is an inexact fix though, because it doesn't take
  account of fact that qcow2 metadata can takes some MBs of space.

  Thus, it would like to see preallocation=full supported in combination
  with backing files.

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