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Re: [Qemu-devel] Re: Strategic decision: COW format

From: Chunqiang Tang
Subject: Re: [Qemu-devel] Re: Strategic decision: COW format
Date: Mon, 14 Mar 2011 10:30:49 -0400

> On Mon, Mar 14, 2011 at 1:53 PM, Chunqiang Tang <address@hidden> 
> > Therefore, during normal execution of a
> > VM, images with snapshots are as fast as images without snapshot.
> Hang on, an image with a snapshot still needs to do copy-on-write,
> just like backing files.  The cost of copy-on-write is reading data
> from the backing file, whereas a non-CoW write doesn't need to do
> that.
> So no, snapshots are not free during normal execution.

You are right. For any implementation of snapshot (internal or external), 
this CoW overhead is unavoidable. What I meant to say was that, other than 
this mandatory CoW overhead, FVD's internal snapshot does not incur any 
additional metadata update overhead (unlike that in QCOW2). 

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