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

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] Re: Strategic decision: COW format
Date: Mon, 14 Mar 2011 14:31:02 +0000

On Mon, Mar 14, 2011 at 2:25 PM, Chunqiang Tang <address@hidden> wrote:
>> 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.

Do you want to send out a break-down of the steps (and cost) involved in doing:

1. Snapshot creation.
2. Snapshot deletion.
3. Opening an image with n snapshots.


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