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Date: Fri, 10 Oct 2008 15:20:43 +0200
From: Avi Kivity <avi@redhat.com>
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Subject: Re: [Qemu-devel] [RFC] Disk integrity in QEMU
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To: Paul Brook <paul@codesourcery.com>
Cc: Chris Wright <chrisw@redhat.com>, Mark McLoughlin <markmc@redhat.com>, kvm-devel <kvm-devel@lists.sourceforge.net>, Laurent Vivier <Laurent.Vivier@bull.net>, qemu-devel@nongnu.org, Ryan Harper <ryanh@us.ibm.com>

Paul Brook wrote:
>>> Absoutely agree that the default should be safe. I don't have enough 
>>> knowledge to say whether O_DIRECT/O_DSYNC is best - which also implies
>>> we should choose the best setting by default, because we can't expect
>>> users to know the tradeoffs either.
>>>       
>> The fact that there are different use models for qemu implies that the
>> default must be chosen at some higher level than qemu code itself.  It
>> might be done using /etc/qemu or ~/.qemu, or at the management
>> interface, but there is no best setting for qemu itself.
>>     
>
> This suggests that the most appropriate defaults are for the users that are 
> least likely to be using a management tool.  I'd guess that the server 
> partitioning folks are most likely to be using a management tool, so qemu 
> defaults should be setup for casual/development use. I don't have hard data 
> to back this up though.
>   

I agree (as my own uses are of the development kind).  That rules out
O_DIRECT as the qemu-level default.  However I'm not sure writeback is a
good default, it's too risky (though I've never been bitten; and I've
had my share of host crashes).

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.