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

[Kevin Wolf]

Am 28.09.2015 um 22:31 hat Eric Blake geschrieben:
> On 09/28/2015 08:13 AM, Paolo Bonzini wrote:
> > 
> > 
> > On 28/09/2015 05:29, Jeff Cody wrote:
> >> This only occurs under two conditions:
> >>
> >>     1. 'mode' != "existing"
> >>     2. bdrv_has_zero_init(target) == NULL
> >>
> > 
> > I'm not sure if mode != "existing" actually matters.  I think what
> > actually matters is sync == "full".
> 
> When mode == 'existing' for a shallow mirror (sync != 'full'), that is
> the caller stating that the guest-visible contents of the destination
> match the guest-visible contents of the backing image.  The only sectors
> to be copied are those that differ from the backing file, and we should
> not be zeroing unrelated sectors because the user has already promised
> they have the same guest-visible content as the backing image would report.

Where is this promise documented? I wasn't aware of it and can't seem to
find it in the QAPI documentation of drive-mirror.

> When mode == 'existing' for a full mirror (sync == 'full'), that is the
> caller stating that they want every single sector of the destination
> written to hold the current state of the source (of course, allowing for
> optimizations such as skipping the write where the contents will read
> back the same as if the write had been performed).
> 
> I think Paolo is right: we care about zeroing unallocated sectors for
> sync == 'full', regardless of whether mode == 'existing'.

I agree.

> I also think the reason Jeff confused it for mode == 'existing' is that
> the other modes let qemu create the file, but qemu does not create block
> devices (the only way to mirror to a block device is via mode ==
> 'existing'), and it is primarily block devices where zero init is not
> guaranteed.

'qemu-img create' works on block devices (even though for raw it doesn't
do more than checking if it's large enough; but for qcow2, it's obvious
that it's necessary), so I'm pretty sure that mode != 'existing' works
on them as well.

Kevin

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