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Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics |
Date: |
Wed, 09 Jan 2013 16:10:32 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/17.0 Thunderbird/17.0 |
Il 09/01/2013 14:04, Liu Yuan ha scritto:
> > > 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.
They get a writeback cache implementation-wise, but they get a
writethrough cache safety-wise. How the cache is implemented doesn't
matter, as long as it "looks like" a writethrough cache.
In fact, consider a local disk that doesn't support FUA. In old QEMU,
images used to be opened with O_DSYNC and that splits each write into
WRITE+FLUSH, just like new QEMU. All that changes is _where_ the
flushes are created. Old QEMU changes it in the kernel, new QEMU
changes it in userspace.
> 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.
It's not by magic. It's by explicitly requesting the disk to do this.
Perhaps it's a bug that the cache mode is not reset when the machine is
reset. I haven't checked that, but it would be a valid complaint.
Paolo
>
> So Guest-WCE only works when users provide us a writeback capable disk.
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, (continued)
- 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, 2013/01/09
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics,
Paolo Bonzini <=
- 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
- Re: [Qemu-devel] [PATCH] sheepdog: implement direct write semantics, Jamie Lokier, 2013/01/10