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Re: [Qemu-devel] [RFC] Disk integrity in QEMU

From: Avi Kivity
Subject: Re: [Qemu-devel] [RFC] Disk integrity in QEMU
Date: Fri, 10 Oct 2008 15:20:43 +0200
User-agent: Thunderbird (X11/20080723)

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 

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