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Re: [Qemu-devel] Re: [PATCH 3/3] disk: don't read from disk until the gu
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] Re: [PATCH 3/3] disk: don't read from disk until the guest starts |
Date: |
Mon, 13 Sep 2010 20:29:43 +0100 |
On Mon, Sep 13, 2010 at 3:13 PM, Kevin Wolf <address@hidden> wrote:
> Am 13.09.2010 15:42, schrieb Anthony Liguori:
>> On 09/13/2010 08:39 AM, Kevin Wolf wrote:
>>>> Yeah, one of the key design points of live migration is to minimize the
>>>> number of failure scenarios where you lose a VM. If someone typed the
>>>> wrong command line or shared storage hasn't been mounted yet and we
>>>> delay failure until live migration is in the critical path, that would
>>>> be terribly unfortunate.
>>>>
>>> We would catch most of them if we try to open the image when migration
>>> starts and immediately close it again until migration is (almost)
>>> completed, so that no other code can possibly use it before the source
>>> has really closed it.
>>>
>>
>> I think the only real advantage is that we fix NFS migration, right?
>
> That's the one that we know about, yes.
>
> The rest is not a specific scenario, but a strong feeling that having an
> image opened twice at the same time feels dangerous. As soon as an
> open/close sequence writes to the image for some format, we probably
> have a bug. For example, what about this mounted flag that you were
> discussing for QED?
There is some room left to work in, even if we can't check in open().
One idea would be to do the check asynchronously once I/O begins. It
is actually easy to check L1/L2 tables as they are loaded.
The only barrier relationship between I/O and checking is that an
allocating write (which will need to update L1/L2 tables) is only
allowed after check completes. Otherwise reads and non-allocating
writes may proceed while the image is not yet fully checked. We can
detect when a table element is an invalid offset and discard it.
Stefan
- [Qemu-devel] Re: [PATCH 3/3] disk: don't read from disk until the guest starts, (continued)
- [Qemu-devel] Re: [PATCH 3/3] disk: don't read from disk until the guest starts, Kevin Wolf, 2010/09/13
- [Qemu-devel] Re: [PATCH 3/3] disk: don't read from disk until the guest starts, Anthony Liguori, 2010/09/13
- [Qemu-devel] Re: [PATCH 3/3] disk: don't read from disk until the guest starts, Kevin Wolf, 2010/09/13
- [Qemu-devel] Re: [PATCH 3/3] disk: don't read from disk until the guest starts, Anthony Liguori, 2010/09/13
- [Qemu-devel] Re: [PATCH 3/3] disk: don't read from disk until the guest starts, Kevin Wolf, 2010/09/13
- Re: [Qemu-devel] Re: [PATCH 3/3] disk: don't read from disk until the guest starts, Anthony Liguori, 2010/09/13
- Re: [Qemu-devel] Re: [PATCH 3/3] disk: don't read from disk until the guest starts, Avi Kivity, 2010/09/14
- Re: [Qemu-devel] Re: [PATCH 3/3] disk: don't read from disk until the guest starts, Anthony Liguori, 2010/09/14
- Re: [Qemu-devel] Re: [PATCH 3/3] disk: don't read from disk until the guest starts, Avi Kivity, 2010/09/14
- Re: [Qemu-devel] Re: [PATCH 3/3] disk: don't read from disk until the guest starts,
Stefan Hajnoczi <=
- Re: [Qemu-devel] Re: [PATCH 3/3] disk: don't read from disk until the guest starts, Kevin Wolf, 2010/09/13
- Re: [Qemu-devel] Re: [PATCH 3/3] disk: don't read from disk until the guest starts, Anthony Liguori, 2010/09/13
- Re: [Qemu-devel] Re: [PATCH 3/3] disk: don't read from disk until the guest starts, Kevin Wolf, 2010/09/14
- [Qemu-devel] Re: [PATCH 3/3] disk: don't read from disk until the guest starts, Juan Quintela, 2010/09/15
[Qemu-devel] [PATCH 1/3] block: allow migration to work with image files, Anthony Liguori, 2010/09/11