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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 10:25:51 -0400

> IIUC, he already uses a refcount table. Actually, I think that a
> refcount table is a requirement to provide the interesting properties
> that internal snapshots have (see my other mail).
> Refcount tables aren't a very complex thing either. In fact, it makes a
> format much simpler to have one concept like refcount tables instead of
> adding another different mechanism for each new feature that would be
> natural with refcount tables.
> The only problem with them is that they are metadata that must be
> updated. However, I think we have discussed enough how to avoid the
> greatest part of that cost.

FVD's novel uses of the reference count table reduces the metadata update 
overhead down to literally zero during normal execution of a VM. This gets 
the bests of QCOW2's reference count table but without its oeverhead. In 
FVD, the reference count table is only updated when creating a new 
snapshot or deleting an existing snapshot. The reference count table is 
never updated during normal execution of a VM.

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