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[Qemu-devel] [Bug 1538541] Re: qcow2 rejects request to use preallocatio
From: |
Max Reitz |
Subject: |
[Qemu-devel] [Bug 1538541] Re: qcow2 rejects request to use preallocation with backing file |
Date: |
Wed, 27 Jan 2016 16:35:28 -0000 |
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.
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