Re: [Qemu-devel] [PATCH 3/3] block: mirror - zero unallocated target sectors when zero init not present

[Kevin Wolf <kwolf@redhat.com>]

Am 30.09.2015 um 16:43 hat Jeff Cody geschrieben:
> On Tue, Sep 29, 2015 at 12:52:33PM +0200, Paolo Bonzini wrote:
> > 
> > 
> > On 29/09/2015 11:35, Kevin Wolf wrote:
> > > The caller could be copying the backing file in the background and it
> > > may not yet be finished.
> > 
> > Yes, and this is permitted (the destination file of mirroring is opened
> > with BDRV_O_NO_BACKING).
> > 
> > Some more assumptions arise when block-job-complete is invoked, because
> > at this point the content must not change under the guest's feet.
> > Because block-job-complete does bdrv_open_backing_file on the
> > destination, for sync!='full' it means that either 1) the image has no
> > backing file, but it starts with the content of the backing file or 2)
> > the image's backing file is complete at the time block-job-complete is
> > invoked.
> > 
> > For mode!='existing' it is always case (2), and the backing file is
> > complete all the time; for mode=='existing' the backing file could be
> > copied in the background, and case (1) could happen as well.  An example
> > of case (1) is replacing sync=='full' with a "fast copy" of the backing
> > file (e.g. via btrfs's COW copies) and sync=='top'.  This should be valid.
> 
> One issue is that QEMU will do mode!='existing' && sync!='full' for
> drivers that do not support backing files (raw host devices, for
> instance).  We could refuse to start a mirror in the case of:
> 
>     mode != 'existing' && sync != 'full' && !target->drv->supports_backing
> 
> Alternatively, we could do the two-pass zero approach in this patch,
> except under the following conditions:
> 
>     sync == 'full' || (mode != 'existing' && !target->drv->supports_backing)
> 
> (In the sync == 'full' case, we could also just queue all sectors, as
> Kevin suggested)

I don't think that mode == 'existing' should play any role in the
behaviour of any block job. There's no reason why doing an external
'qemu-img create' should make it do anything different compared to
images created using the monitor.

Kevin