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Re: [Xen-devel] Shouldn't backend devices for VMX domain disks be opened

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Subject: Re: [Xen-devel] Shouldn't backend devices for VMX domain disks be opened with O_DIRECT?
From: Luciano Miguel Ferreira Rocha <strange@xxxxxxxxxxxxx>
Date: Fri, 3 Feb 2006 00:31:18 +0000
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On Thu, Feb 02, 2006 at 06:09:11PM -0600, Anthony Liguori wrote:
> >I agree. It sounds like a correctness problem. It's just like disks
> >with write caching enabled.  
> > 
> >
> Referring to the original question, which has been quoted away, 
> journaling doesn't require that data be written to disk per-say but that 
> writes occur in a particular order.  A journal is always recoverable 
> given that writes occur in the expected order.  A buffer cache will have 
> no effect on that order so you're no more likely to have corruption than 
> if you disabled the buffer cache.

Corruption meaning that the domU thinks data has been committed to disk
but never has (dom0 crashed before the cache could be flushed).

The correctness of some protocols or procedures depend on being able to
forcefully commit changes to disk (databases, for example).

> If anything, what you really want (from a performance perspective) is to 
> disable the buffer cache in the domU and leave it enabled in the dom0 
> (this is what the paravirtual drivers should be doing IIRC).

I disagree. domU must be able to sync(). And if domUs are already
caching data, why let them pollute dom0's cache?


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