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

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

[Daniel Berrange]

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.

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

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

[Max Reitz]

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).

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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.

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

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

[Daniel Berrange]

> 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.

This sounds like a bit of extra work, but I'm puzzelled why we can't
have a preallocation option which simply calls fallocate() to grow the
file to the right size. From qcow2 pov, the extra clusters won't be
allocated - we're just making sure the filesystem has reserved
sufficient space for when qcow2 does later allocate the clusters during
a copy-on-write.  Perhaps this would imply a new option to the
'preallocation' option rather than 'full'

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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.

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

[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.

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

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

[Daniel Berrange]

Preallocating the understanding image file at the management layer
though means that the mgmt tool has to understand the full details of
the qcow2 file format. The mgmt layer knows it created a 20 GB qcow2
file, but does not know that once you have stored 20 GB of user data in
it, it will actually consume  20 GB + NNN MB extra. Calculating hat NNN
MB extra is trivial for qemu since it knows the file format already, but
hard for the mgmt app

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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

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

[Max Reitz]

It is not exactly trivial, and it being in qemu does not make it
simpler.
http://git.qemu.org/?p=qemu.git;a=blob;f=block/qcow2.c;h=fd8436c5f8b13ab0e8c605147ce76e6b6a8e5f95;hb=HEAD#l2105
is the code that calculates the size; as you can see, it is pretty self-
contained.

Max

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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

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

[Thomas Huth]

The QEMU project is currently considering to move its bug tracking to
another system. For this we need to know which bugs are still valid
and which could be closed already. Thus we are setting older bugs to
"Incomplete" now.

If you still think this bug report here is valid, then please switch
the state back to "New" within the next 60 days, otherwise this report
will be marked as "Expired". Or please mark it as "Fix Released" if
the problem has been solved with a newer version of QEMU already.

Thank you and sorry for the inconvenience.


** Changed in: qemu
       Status: New => Incomplete

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

Title:
  qcow2 rejects request to use preallocation with backing file

Status in QEMU:
  Incomplete

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

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

[Launchpad Bug Tracker]

[Expired for QEMU because there has been no activity for 60 days.]

** Changed in: qemu
       Status: Incomplete => Expired

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Title:
  qcow2 rejects request to use preallocation with backing file

Status in QEMU:
  Expired

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