[Qemu-devel] Fw:Re:What does "COW" mean?

[Qemu-devel] Fw:Re:What does "COW" mean?

[shhuiw]

Hi,

Sorry to trouble you.

I posted my question in the user mailing list, but can not get the right answer.
So I forward it to the devel mailing list. Hope to find answer here.


At 2014-09-02 04:33:50, "shhuiw" <shhuiw@163.com> wrote:
>
>
>Hi,
>
>I'm new to qemu community, and I'm trying the COW image format (old but simple:-).
>I have read through its source code, and didn't find anything about 'copy on write'.
>I wonder wthat "COW" stands for?

Sorry for my unclear expression.
I mean when copy-on-write happens if COW image format is used, and how the COW driver code handles cop-on-write?

>
>
>--
>
>Regards,
>shhuiw


--

Regards,
shhuiw

Re: [Qemu-devel] Fw:Re:What does "COW" mean?

[Stefan Hajnoczi]

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On Wed, Sep 03, 2014 at 12:47:44PM +0800, shhuiw wrote:
> At 2014-09-02 04:33:50, "shhuiw" <shhuiw@163.com> wrote:
> >
> >
> >Hi,
> >
> >I'm new to qemu community, and I'm trying the COW image format (old but simple:-).
> >I have read through its source code, and didn't find anything about 'copy on write'.
> >I wonder wthat "COW" stands for?
> 
> Sorry for my unclear expression.
> I mean when copy-on-write happens if COW image format is used, and how the COW driver code handles cop-on-write?

It doesn't really "copy on write" but it is a sparse overlay.  Only
written sectors need to be allocated in the image file, other sectors
will be read from the backing file.

Stefan

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