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Re: [Qemu-block] [Qemu-devel] [PATCH v3 09/16] block/qcow2: Generalize p

From: Eric Blake
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH v3 09/16] block/qcow2: Generalize preallocate()
Date: Tue, 30 May 2017 16:26:18 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.1.0

On 05/26/2017 11:55 AM, Max Reitz wrote:
> This patch adds two new parameters to the preallocate() function so we
> will be able to use it not just for preallocating a new image but also
> for preallocated image growth.
> The offset parameter allows the caller to specify a virtual offset from
> which to start preallocating. For newly created images this is always 0,
> but for preallocating growth this will be the old image length.
> The new_length parameter specifies the supposed new length of the image
> (basically the "end offset" for preallocation). During image truncation,
> bdrv_getlength() will return the old image length so we cannot rely on
> its return value then.

bdrv_getlength() is (currently) always sector-aligned (rounding up as

new_length is passed from qcow2_create2()'s total_size, which in turn
comes from qcow2_create()'s size, which can be user-supplied - but also
appears that we round up to ensure it is always sector-aligned.

Testing: 'qemu-img create -f qcow2 a.img 1' reports "size=1", but
'qemu-img info a.img' reports "virtual size: 512" - so we have a
secondary bug worth fixing later in that we are rounding AFTER what we
report to the user, but I'm not seeing a behavior change in this patch.

> Signed-off-by: Max Reitz <address@hidden>
> ---
>  block/qcow2.c | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)

Reviewed-by: Eric Blake <address@hidden>

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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