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Re: [Qemu-devel] Re: Strategic decision: COW format
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] Re: Strategic decision: COW format |
Date: |
Mon, 14 Mar 2011 15:34:11 +0100 |
User-agent: |
Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.15) Gecko/20101027 Fedora/3.0.10-1.fc12 Thunderbird/3.0.10 |
Am 14.03.2011 15:25, schrieb Chunqiang Tang:
>> 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.
Yeah, I think that's basically an interesting property. However, I don't
think that it makes a big difference compared to qcow2's refcount table
when you use a writeback metadata cache.
What about the question that I had in my other mail? (How do you
determine if a cluster is free without scanning the whole lookup table?)
I think this might be the missing piece for me to understand how your
approach works.
Kevin
- Re: [Qemu-devel] Re: Strategic decision: COW format, (continued)
- Re: [Qemu-devel] Re: Strategic decision: COW format, Anthony Liguori, 2011/03/14
- Re: [Qemu-devel] Re: Strategic decision: COW format, Kevin Wolf, 2011/03/14
- Re: [Qemu-devel] Re: Strategic decision: COW format, Chunqiang Tang, 2011/03/14
- Re: [Qemu-devel] Re: Strategic decision: COW format, Anthony Liguori, 2011/03/14
- Re: [Qemu-devel] Re: Strategic decision: COW format, Stefan Hajnoczi, 2011/03/14
- Re: [Qemu-devel] Re: Strategic decision: COW format, Kevin Wolf, 2011/03/14
- Re: [Qemu-devel] Re: Strategic decision: COW format, Stefan Hajnoczi, 2011/03/14
- Re: [Qemu-devel] Re: Strategic decision: COW format, Chunqiang Tang, 2011/03/14
- Re: [Qemu-devel] Re: Strategic decision: COW format, Kevin Wolf, 2011/03/14
- Re: [Qemu-devel] Re: Strategic decision: COW format, Chunqiang Tang, 2011/03/14
- Re: [Qemu-devel] Re: Strategic decision: COW format,
Kevin Wolf <=
- Re: [Qemu-devel] Re: Strategic decision: COW format, Stefan Hajnoczi, 2011/03/14
- Re: [Qemu-devel] Re: Strategic decision: COW format, Chunqiang Tang, 2011/03/14
- Re: [Qemu-devel] Re: Strategic decision: COW format, Kevin Wolf, 2011/03/14
- Re: [Qemu-devel] Re: Strategic decision: COW format, Chunqiang Tang, 2011/03/14
- Re: [Qemu-devel] Re: Strategic decision: COW format, Kevin Wolf, 2011/03/14
- Message not available
- Re: [Qemu-devel] Re: Strategic decision: COW format, Chunqiang Tang, 2011/03/14
- Re: [Qemu-devel] Re: Strategic decision: COW format, Anthony Liguori, 2011/03/14
- Re: [Qemu-devel] Re: Strategic decision: COW format, Kevin Wolf, 2011/03/14
- Re: [Qemu-devel] Re: Strategic decision: COW format, Anthony Liguori, 2011/03/14
- Re: [Qemu-devel] Re: Strategic decision: COW format, Chunqiang Tang, 2011/03/14