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Re: [Qemu-devel] KVM call agenda for October 25
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] KVM call agenda for October 25 |
Date: |
Tue, 25 Oct 2011 17:32:06 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:7.0) Gecko/20110927 Thunderbird/7.0 |
Am 25.10.2011 16:06, schrieb Anthony Liguori:
> On 10/25/2011 08:56 AM, Kevin Wolf wrote:
>> Am 25.10.2011 15:05, schrieb Anthony Liguori:
>>> On 10/25/2011 07:35 AM, Kevin Wolf wrote:
>>>> Am 24.10.2011 13:35, schrieb Paolo Bonzini:
>>>>> On 10/24/2011 01:04 PM, Juan Quintela wrote:
>>>>>>
>>>>>> Hi
>>>>>>
>>>>>> Please send in any agenda items you are interested in covering.
>>>>>
>>>>> - What's left to merge for 1.0.
>>>>
>>>> I would still like to cache the default cache mode (probably to
>>>> cache=writeback). We don't allow guests to toggle WCE yet which Anthony
>>>> would have liked to see before doing the change. Is it a strict
>>>> requirement?
>>>
>>> I don't see a way around it. If the default mode is cache=writeback, then
>>> we're
>>> open to data corruption in any guest where barrier=0. With guest togglable
>>> WCE,
>>> it ends up being a guest configuration issue so we can more or less defer
>>> responsibility.
>>
>> So do you think that offering a WCE inside the guest would be a real
>> solution or just a way to have an excuse?
>
> No, it offers a mechanism to "fix mistakes" at run-time verses at start up
> time.
This is true (in both directions). But I think it's independent from the
right default.
> It also means that you can make template images that understand that they
> don't support barriers and change the WCE setting appropriately.
Isn't that really a job for management tools?
>> Christoph said that OSes don't usually change this by themselves, it
>> would need an administrator manually changing the setting. But if we
>> require that, we can just as well require that the administrator set
>> cache=writethrough on the qemu command line.
>
> The administrator of the guest != the administrator of the host.
But the administrator of the guest == the owner of the qemu instance,
no? He should be the one to use the management tools and configure his VMs.
>>> Do you think it's a good idea to change the default mode w/o guest WCE
>>> toggle
>>> support? What's your view about older guests if we change the default mode?
>>> What's your main motivation for wanting to change the default mode?
>>
>> Because people are constantly complaining about the awful
>> (cache=writethrough) performance they get before they are told they
>> should use a different cache option. And they are right. The
>> out-of-the-box experience with qemu's block performance really sucks.
>
> With qcow2 you mean, right?
No, with any format, including raw. Which isn't surprising at all,
O_SYNC makes writes very expensive.
>>> I'd be much more open to changing the default mode to cache=none FWIW since
>>> the
>>> risk of data loss there is much, much lower.
>>
>> I think people said that they'd rather not have cache=none as default
>> because O_DIRECT doesn't work everywhere.
>
> Where doesn't it work these days? I know it doesn't work on tmpfs. I know
> it
> works on ext[234], btrfs, nfs.
I think tmpfs was named (and failing to start with default settings on
tmpfs would be nasty enough), but iirc Alex had another one.
Kevin
- Re: [Qemu-devel] KVM call agenda for October 25, (continued)
- Re: [Qemu-devel] KVM call agenda for October 25, Peter Maydell, 2011/10/24
- Re: [Qemu-devel] KVM call agenda for October 25, Kevin Wolf, 2011/10/25
- Re: [Qemu-devel] KVM call agenda for October 25, Kevin Wolf, 2011/10/25
- Re: [Qemu-devel] KVM call agenda for October 25, Anthony Liguori, 2011/10/25
- Re: [Qemu-devel] KVM call agenda for October 25,
Kevin Wolf <=
- Re: [Qemu-devel] KVM call agenda for October 25, Alexander Graf, 2011/10/25
- Re: [Qemu-devel] KVM call agenda for October 25, Anthony Liguori, 2011/10/26
- Re: [Qemu-devel] KVM call agenda for October 25, Kevin Wolf, 2011/10/26
- Re: [Qemu-devel] KVM call agenda for October 25, Markus Armbruster, 2011/10/26
- Re: [Qemu-devel] KVM call agenda for October 25, Paolo Bonzini, 2011/10/26
- Re: [Qemu-devel] KVM call agenda for October 25, Markus Armbruster, 2011/10/26
- Re: [Qemu-devel] KVM call agenda for October 25, Daniel P. Berrange, 2011/10/26
- Re: [Qemu-devel] KVM call agenda for October 25, Kevin Wolf, 2011/10/26
- Re: [Qemu-devel] KVM call agenda for October 25, Daniel P. Berrange, 2011/10/26
- Re: [Qemu-devel] KVM call agenda for October 25, Kevin Wolf, 2011/10/26