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Re: [Qemu-devel] Re: [patch 2/3] Add support for live block copy

From: Anthony Liguori
Subject: Re: [Qemu-devel] Re: [patch 2/3] Add support for live block copy
Date: Wed, 02 Mar 2011 10:07:53 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20101027 Lightning/1.0b1 Thunderbird/3.0.10

On 03/02/2011 08:00 AM, Avi Kivity wrote:
On 03/02/2011 02:39 PM, Anthony Liguori wrote:

Here is where your race is:

2. Management sends a switch command

3. QEMU receives switch command

4. QEMU stops doubling IO and switches to the destination

5. QEMU sends acknowledgement of switch command

6. Management receives acknowledge of switch command

7. Management changes internal state definition to reflect the new destination

If QEMU or the management tool crashes after step 4 and before step 6, when the management tool restarts QEMU with the source image, data loss will have occurred (and potentially corruption if a flush had happened).

No. After step 2, any qemu restart will be with the destination image. If the management tool restarts, it can query the state (or just re-issue the switch command, which is idempotent).

Yeah, I think you're right, although I need to think through it a bit more.


Anthony Liguori

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