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

[Max Reitz <1538541@bugs.launchpad.net>]

Using any preallocation value other than none will result in all data
clusters of the new image being used. That means that any I/O request
will be served by that image, and never by the backing file. This is why
preallocating an image with a backing file is not supported, because it
generally doesn't make any sense. The backing file will never be seen
anyway.

In order to support this, qcow2 will need to support preallocated data
clusters which are explicitly marked as empty (where "empty" is not
"zero"; "empty" means "fall through to the backing file"). This has been
proposed before, but has not been implemented so far.

By the way, this is the very reason why explicitly forbidding the
combination of backing file and preallocation is very reasonable: Right
now, the backing file would be invisible, a preallocated image always
returns zeros when read. With the above feature implemented, the backing
file would be visible. In order to allow this change in behavior, we
have to make the combination an error for now.

Max

PS: The reason I write this is so that you know that this is not a bug,
but correct behavior in view of a missing feature (that should indeed be
implemented).

-- 
You received this bug notification because you are a member of qemu-
devel-ml, which is subscribed to QEMU.
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.

To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1538541/+subscriptions