Re: [Qemu-discuss] Converting qcow2 image on the fly to raw format

[Nir Soffer]

On Mon, Jul 9, 2018 at 8:06 PM Richard W.M. Jones <address@hidden> wrote:

> On Mon, Jul 09, 2018 at 07:02:50PM +0200, Kevin Wolf wrote:
> > Am 09.07.2018 um 18:52 hat Richard W.M. Jones geschrieben:
> > > On Mon, Jul 09, 2018 at 07:38:05PM +0300, Nir Soffer wrote:
> > > > We are discussing importing VM images to KubVirt. The goal is to be
> > > > able to import an existing qcow2 disk, probably some appliance stored
> > > > on http server, and and convert it to raw format for writing to
> storage.
> > > >
> > > > This can be also useful for for oVirt for importing OVA, since we
> like to
> > > > pack
> > > > disks in qcow2 format inside OVA, but the user may like to use raw
> disks, or
> > > > for uploading existing disks.
> > > >
> > > > Of course converting the image using qemu-img is easy, but requires
> > > > downloading the image to temporary disk. We would like to avoid
> temporary
> > > > disks, or telling users to convert the image.
> > > >
> > > > Base on the discussion we had here:
> > > >
> https://lists.ovirt.org/archives/list/address@hidden/thread/GNAVJ253FP65QUSOONES5XZGRIDX5ABC/#YMLSEGU7PN3MX5MUORGEGGAQLLSL4KKJ
> > > >
> > > > I think this is impossible since qcow2 is not built for streaming.
> But both
> > > > Richard and Eric suggested some solutions.
> > > >
> > > > The flow is:
> > > >
> > > >     qcow2 image -- http --> importer -> raw file
> > > >
> > > > Is it possible to implement the importer using qemu-img and qemu-nbd,
> > > > or maybe nbdkit?
> > >
> > > Strictly speaking streaming qcow2 to raw is not possible.  However
> > > placing an overlay on top of the original remote image will allow
> > > streaming to raw with only a modest amount of local storage consumed.
> > >
> > > You can demonstrate this fairly easily:
> > >
> > > $ qemu-img create -f qcow2 -b 'json: { "file.driver": "https",
> "file.url": "
> https://uk-mirrors.evowise.com/fedora/releases/28/Cloud/x86_64/images/Fedora-Cloud-Base-28-1.1.x86_64.qcow2";,
> "file.timeout": 10000 }' /var/tmp/overlay.qcow2
> > > $ qemu-img convert -p -f qcow2 -O raw overlay.qcow2 fedora.img
> >
> > This overlay stays empty, so it's pretty pointless and you could just
> > directly point 'qemu-img convert' to https and the real image.
>
> Right, indeed.  I was copying what virt-v2v does without thinking
> about it enough.  virt-v2v needs the overlay because it actually wants
> to write into it, and it does copy-on-read for the first phase (not
> the final ‘qemu-img convert’).
>

Thanks, I just tested the simple:

    qemu-img convert -p -f qcow2 -O raw http://localhost/orig.qcow2
converted.raw

And it just works :-)

I got timeouts trying to download from
https://download.fedoraproject.org/pub/alt/atomic/stable/Fedora-Atomic-28-20180625.1/AtomicHost/x86_64/images/Fedora-AtomicHost-28-20180625.1.x86_64.qcow2

I guess we need to use
'json: { "file.driver": "http", "file.url": "url...", "file.timeout": 10000
}'
To change timeout? Where is these and other options documented?

I did also some timings, using sever on local network with 1g nic.

$ time wget http://local.server/Fedora-AtomicHost-28-20180625.1.x86_64.qcow2

...
Length: 638043136 (608M) [application/octet-stream]Saving to:
‘Fedora-AtomicHost-28-20180625.1.x86_64.qcow2’

Fedora-AtomicHost-28-20180625.1.x86_64.qcow2
100%[=====================================================================================================>]
608.49M   107MB/s    in 5.6s

2018-07-09 21:38:39 (108 MB/s) -
‘Fedora-AtomicHost-28-20180625.1.x86_64.qcow2’ saved [638043136/638043136]


real 0m5.941s
user 0m0.183s
sys 0m1.185s

$ time qemu-img convert -p -f qcow2 -O raw
http://local.server/Fedora-AtomicHost-28-20180625.1.x86_64.qcow2
converted.raw
    (100.00/100%)

real 0m14.217s
user 0m5.235s
sys 0m2.343s

$ time qemu-img convert -p -f qcow2 -O raw
Fedora-AtomicHost-28-20180625.1.x86_64.qcow2 converted.raw
    (100.00/100%)

real 0m11.909s
user 0m4.728s
sys 0m1.595s

So converting on the fly is even little faster then downloading to
temporary file and converting.

Nir