Re: [Qemu-devel] [PATCH v7 0/4] Add shrink image for qcow2

[Max Reitz]

On 2017-08-22 01:31, John Snow wrote:
> 
> 
> On 08/17/2017 05:15 AM, Pavel Butsykin wrote:
>> This patch add shrinking 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 shrink is done by punching 
>> holes
>> in the image file.
>>
>> # ./qemu-img create -f qcow2 image.qcow2 4G
>> 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
>> wrote 1073741824/1073741824 bytes at offset 0
>> 1 GiB, 1 ops; 0:00:04.59 (222.886 MiB/sec and 0.2177 ops/sec)
>>
>> # ./qemu-img resize image.qcow2 512M
>> warning: qemu-img: Shrinking an image will delete all data beyond the 
>> shrunken image's end. Before performing such an operation, make sure there 
>> is no important data there.
>> error: qemu-img: Use the --shrink option to perform a shrink operation.
>>
>> # ./qemu-img resize --shrink image.qcow2 128M
>> Image resized.
>>
>> # ./qemu-img info image.qcow2
>> image: image.qcow2
>> file format: qcow2
>> virtual size: 128M (134217728 bytes)
>> disk size: 128M
>> cluster_size: 65536
>> Format specific information:
>>     compat: 1.1
>>     lazy refcounts: false
>>     refcount bits: 16
>>     corrupt: false
>>
>> # du -h image.qcow2
>> 129M    image.qcow2
>>
> 
> It looks sane to me, and already has a full set of R-Bs from Max. Are we
> waiting for Kevin?

We were waiting for Kevin to handle 2.10 patches so he could go into PTO
and for me to come back from PTO. ;-)

I'm still sifting through my inbox... When I'm done, I'll take a look,
but I have no idea how long that might last.
(Since 2.10 isn't out yet, there might be more pressing issues still...?
I don't quite now yet, to be honest...)

> It looks like in Vladimir's series we sidestepped the problem of what
> happens if we resize with persistent bitmaps: we deny the operation,
> regardless of if we are shrinking, growing, or have bitmaps that are
> read-only, frozen, or what-have-you.
> 
> It shouldn't be too hard to add soon, but this is fine for now.
> (I think for adding it we just need to make sure the bitmaps aren't
> frozen and are not read-only, and the implementation bitmap structure
> can already handle truncation in either direction just fine.)
> 
> Anyway;
> 
> Reviewed-by: John Snow <address@hidden>

Thanks, in any case. :-)

Max

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