From: Max Reitz <1538541@bugs.launchpad.net>
To: qemu-devel@nongnu.org
Subject: [Qemu-devel] [Bug 1538541] Re: qcow2 rejects request to use preallocation with backing file
Date: Fri, 29 Jan 2016 13:02:02 -0000 [thread overview]
Message-ID: <20160129130202.15839.90307.malone@gac.canonical.com> (raw)
In-Reply-To: 20160127125558.31349.12989.malonedeb@gac.canonical.com
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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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
next prev parent reply other threads:[~2016-01-29 13:11 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-01-27 12:55 [Qemu-devel] [Bug 1538541] [NEW] qcow2 rejects request to use preallocation with backing file Daniel Berrange
2016-01-27 16:35 ` [Qemu-devel] [Bug 1538541] " Max Reitz
2016-01-27 16:50 ` Daniel Berrange
2016-01-28 13:57 ` Kevin Wolf
2016-01-28 14:19 ` Daniel Berrange
2016-01-29 13:02 ` Max Reitz [this message]
2021-04-22 5:03 ` Thomas Huth
2021-06-22 4:17 ` Launchpad Bug Tracker
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