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Re: [Qemu-block] [PATCH 0/2] Add reduce image for qcow2

From: Eric Blake
Subject: Re: [Qemu-block] [PATCH 0/2] Add reduce image for qcow2
Date: Wed, 31 May 2017 10:03:51 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.1.0

On 05/31/2017 09:43 AM, Pavel Butsykin wrote:
> This patch adds the reduction of the image file for qcow2. As a result, this
> allows us to reduce the virtual image size and free up space on the disk 
> without
> copying the image. Image can be fragmented and reduction is done by punching
> holes in the image file.
> # ./qemu-img create -f qcow2 -o size=4G image.qcow2
> Formatting 'image.qcow2', fmt=qcow2 size=4294967296 encryption=off 
> cluster_size=65536 lazy_refcounts=off refcount_bits=16
> # ./qemu-io -c "write -P 0x22 0 1G" image.qcow2

So this is 1G of guest-visible data...

> # ./qemu-img resize image.qcow2 128M
> Image resized.

...and you are truncating the image by throwing away guest-visible
content, with no warning or double-checking (such as requiring a -f
force parameter or something) about the data loss.  Shrinking images is
something we should allow, but it feels like is a rare enough operation
that you don't want it to be casually easy to throw away data.

Is it feasible to require that a shrink operation will not be performed
unless all clusters being eliminated have been previously discarded (or
maybe written to zero), as an assurance that the guest did not care
about the tail of the image?

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

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