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Date: Mon, 14 Mar 2011 20:43:31 +0530
From: Dushyant Bansal <cs5070214@cse.iitd.ac.in>
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Subject: Re: [Qemu-devel] Re: KVM call agenda for Jan 25
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: qemu-devel@nongnu.org

>
> Nice that qemu-img convert isn't that far out by default on raw :).
>
> About Google Summer of Code, I have posted my take on applying and
> want to share that with you and qemu-devel:
>
> http://blog.vmsplice.net/2011/03/advice-for-students-applying-to-google.html
>    
Thanks for sharing your experiences.

After reading about qcow2 and qed and how they organize data (thanks to 
the newly added qcow2 doc and discussions on the mailing list), this is 
what I understand.

So, the main difference between qed and qcow2 is the absence of 
reference count structure in qed(means less meta data).
It improves performance due to:
1. For write operations, less or no metadata to update.
2. Data write and metadata write can be in any order

This also means these features are no longer supported:
1. Internal snapshots,
2. CPU/device state snapshots,
3. Compression,
4. Encryption

Now, coming to qed<-->qcow2 conversion, I want to clarify some things.

1. header_size: variable in qed, equals to cluster size in qcow2:
When will it be larger than 1 cluster in qed? So, what will happen to 
that extra data on qed->qcow2 conversion.

2. L2 table size: equals to L1 table size in qed, equals to cluster size 
in qcow2:
we need to take it into account during conversion.

3. refcount table:
qcow2->qed:we do not keep refcount structure
qed->qcow2: initialize refcount structure

So, a qcow2->qed->qcow2 conversion means if earlier, qcow2 image was 
using additional features{1-4}, all information related to that will be 
lost.

What do you think? Please correct me if I am making some incorrect 
statement.

Thanks,
Dushyant