Re: [Qemu-devel] Possibly incorrect data sparsification by qemu-img

[Richard W.M. Jones]

On Tue, Apr 23, 2019 at 01:30:28PM +0200, Martin Kletzander wrote:
> I am using qemu-img with nbdkit to transfer a disk image and the
> update it with extra data from newer snapshots.  The end image
> cannot be transferred because the snapshots will be created later
> than the first transfer and we want to save some time up front.  You
> might think of it as a continuous synchronisation.

It's important to note here that Martin is reading the data from a
VMware server, so this is not something that can be solved with qemu's
own snapshots.

[...]

I think the following is an even simpler demo which gets to the nub of
the problem as I understand it:

$ rm -f disk.img snap.img
$ dd if=/dev/urandom of=disk.img bs=2M count=1
$ dd if=/dev/zero of=snap.img bs=2M count=1
$ qemu-img convert -n -p snap.img disk.img
$ hexdump -C disk.img | head
00000000  18 30 e8 1f 09 f0 bb 2c  2f c7 b3 97 8f 12 fe 4b  |.0.....,/......K|
00000010  66 f7 28 cb 8e 72 2a 37  6b fa 98 2e a0 e6 d9 cf  |f.(..r*7k.......|
[etc] <- ie. not zeroes

Should we expect disk.img to contain zeroes at the end?

Rich.

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