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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.
To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1538541/+subscriptions
- [Qemu-devel] [Bug 1538541] [NEW] qcow2 rejects request to use preallocation with backing file,
Daniel Berrange <=
- [Qemu-devel] [Bug 1538541] Re: qcow2 rejects request to use preallocation with backing file, Max Reitz, 2016/01/27
- [Qemu-devel] [Bug 1538541] Re: qcow2 rejects request to use preallocation with backing file, Daniel Berrange, 2016/01/27
- [Qemu-devel] [Bug 1538541] Re: qcow2 rejects request to use preallocation with backing file, Kevin Wolf, 2016/01/28
- [Qemu-devel] [Bug 1538541] Re: qcow2 rejects request to use preallocation with backing file, Daniel Berrange, 2016/01/28
- [Qemu-devel] [Bug 1538541] Re: qcow2 rejects request to use preallocation with backing file, Max Reitz, 2016/01/29