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

[Jeff Cody]

On Sep 28, 2015 4:12 PM, "Paolo Bonzini" <address@hidden> wrote:
>
>

Replying from mobile; please excuse any formatting issues.

>
> 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".
>
> The reasons are:
>
> 1) with sync != "full", unallocated target sectors should remain
> unallocated on the destination because they are supposed to point to the
> backing file.

I guess that makes sense.  What about  the case when the target is a raw device without zero init?  There is no backing file... Of course, perhaps in the raw case the user should be using sync==full anyways.

>
> 2) even with mode == "existing" you expect the data to be consistent at
> the end of the mirroring
>

The reason I added the "existing" exception was so the user could avoid the time penalty of zeroing out the data if they knew the target had already explicitly been zeroed.  Do you think it is fair to assume that if the user specified existing, that they take responsibility for setting up the target image how they like (including data initialization)?  Or should we add another option for mirror, to allow the user to bypass the zero fill?

Thanks,
Jeff