[Qemu-devel] [Bug 1538541] Re: qcow2 rejects request to use preallocation with backing file

[Kevin Wolf]

The idea that qcow2 could just reserve enough space that it will never
need any additional clusters stands on somewhat shaky ground anyway. You
can count in metadata such as refcount tables/blocks and the L1/L2 table
for an image with the full virtual disk size used. This doesn't cover
things like snapshots or in the future bitmaps; I'm not completely sure,
but it might also fail to cover some scenarios that involve discard and
where some space isn't immediately reused due to image fragmentation.
Whether or not a given static size is sufficient for an image depends
primarily on how the image is going to be used.

What you seem to want isn't really qcow2 preallocation (which would
involve, as Max said, preallocating all clusters on the qcow2 level),
but preallocation of the image file in the file system layer to a size
that matches your use of qcow2. I'm afraid that doing this in the
management layer, which actually knows best how it's going to use the
image, makes more sense than letting qemu guess and implement a hack
that preallocates only on the file system, but not the image format
level.

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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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