[pve-devel] [PATCH qemu-server 1/2] Add DiskImport module to import external disk images into a VM

[Wolfgang Bumiller]

On Tue, Apr 25, 2017 at 07:04:31PM +0200, Alexandre DERUMIER wrote:
> >>> +    my $convert_command = ['qemu-img', 'convert', $args->{src_path}, '-p', '-n'];
> >>> +    if ($dst->{has_sparseinit}) {
> >>
> >>'has_sparseinit' does not exist in $dst, i guess 'sparseinit' was meant.
> >>
> >>> +        push @$convert_command, "zeroinit:$dst->{path}";
> >>> +    } else {
> >>> +        push @$convert_command, $dst->{path};
> >>> +    }
> 
> I need to test, but I'm pretty sure that we don't need zeroinit for qemu-img.
> (as qemu-img skip zero, and don't write them on target).
> 
> Currently we don't use it for qemu-img convert in offline vm cloning.
> 
> It's only needed for online qemu drive-mirror.  

It's not specific to online mirroring. The problem is that qemu doesn't
always know whether the _target_ is zero-initialized, in which case it
cannot skip writing zeroes one way or another. The worst case there is
lvm-thin where none of the regular zero-out mechanisms work: BLKDISCARD
says it doesn't zero (even though it does if you match its discard
granularity (which you'd definitely do if you discard the entire device
as initialization), but that's beside the point), fallocate()'s
PUNCH_HOLE doesn't work since it's not a raw file and BLKZEROOUT will
fall back to allocating and writing zeroes as well since it follows the
same rules (it also checks BLKDISCARDZEROES...).