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Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics
From: |
Liu Yuan |
Subject: |
Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics |
Date: |
Wed, 09 Jan 2013 21:04:52 +0800 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/17.0 Thunderbird/17.0 |
On 01/09/2013 08:42 PM, Kevin Wolf wrote:
> Am 09.01.2013 13:07, schrieb Liu Yuan:
>> Besides performance, I think backward compatibility is more important:
>> 1 if we run a old kernel host (quite possible for a long running
>> server) which doesn't support WCE, then we will never have a chance to
>> choose writethrough cache for guest OS against new QEMU (most users tend
>> to update user space tools to exclude bugs)
>
> How does the host kernel even play a role in this context?
>
Oops, my mind was broken. Okay, guest OS which doesn't support WCE can't
change cache type as we discussed.
>> 2 The upper layer software which relies on the 'cache=xxx' to choose
>> cache mode will fail its assumption against new QEMU.
>
> Which assumptions do you mean? As far as I can say the behaviour hasn't
> changed, except possibly for the performance.
>
When users set 'cache=writethrough' to export only a writethrough cache
to Guest, but with new QEMU, it will actually get a writeback cache as
default.
>> We can see that the writeback behavior for Guest-WCE toggling is the
>> same as expected. The difference is that if we set cache=writethrough,
>> guest can't change it via WCE toggling.
>
> Then you need to make sure to communicate this to the guest. I'm not
> convinced that adding extra logic to each device for this is a good idea.
>
We don't need to communicate to the guest. I think 'cache=xxx' means
what kind of cache the users *expect* to export to Guest OS. So if
cache=writethrough set, Guest OS couldn't turn it to writeback cache
magically. This is like I bought a disk with 'writethrough' cache
built-in, I didn't expect that it turned to be a disk with writeback
cache under the hood which could possible lose data when power outage
happened.
So Guest-WCE only works when users provide us a writeback capable disk.
Thanks,
Yuan
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, (continued)
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Paolo Bonzini, 2013/01/09
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Liu Yuan, 2013/01/09
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Paolo Bonzini, 2013/01/09
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Liu Yuan, 2013/01/09
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Liu Yuan, 2013/01/09
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Paolo Bonzini, 2013/01/09
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Liu Yuan, 2013/01/09
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Liu Yuan, 2013/01/09
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Liu Yuan, 2013/01/09
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Kevin Wolf, 2013/01/09
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics,
Liu Yuan <=
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Paolo Bonzini, 2013/01/09
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Liu Yuan, 2013/01/10
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, MORITA Kazutaka, 2013/01/11
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Liu Yuan, 2013/01/11
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Paolo Bonzini, 2013/01/11
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Liu Yuan, 2013/01/11
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Paolo Bonzini, 2013/01/11
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Liu Yuan, 2013/01/11
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Paolo Bonzini, 2013/01/11
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Kevin Wolf, 2013/01/11