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Re: [Qemu-devel] Default cache mode


From: Anthony Liguori
Subject: Re: [Qemu-devel] Default cache mode
Date: Wed, 29 Jun 2011 08:05:12 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.17) Gecko/20110424 Lightning/1.0b2 Thunderbird/3.1.10

On 06/29/2011 08:00 AM, Avi Kivity wrote:
On 06/29/2011 02:59 PM, Kevin Wolf wrote:
Hi,

I think we have touched this topic before during some IRC discussions or
somewhere deep in a mailing list thread, but I think it hasn't been
discussed on the list.

Our default cache mode of cache=writethrough is extremely conservative
and provides absolute safety at the cost of performance, and most people
don't use it if they know that it can be changed because it just
performs too bad. There are use cases where you need it (broken guest
OS), but none and writeback are just as correct with respect to the
specs and they are safe to use with current OSes. And even with broken
OSes, in many use cases it doesn't really matter if you lose a VM and
have to reinstall it (which is probably true even more for users
invoking qemu directly instead of using libvirt).

I think the motivation to switch from writeback to writethrough as
default was that writeback was entirely unsafe back then. This isn't
true any more, so is there still enough reason to have the slow
writethrough mode as default?

I'm not entirely sure if I should suggest writeback or none as the new
default, but I think it could make sense to change it.

So long as -M old retains the old behaviour, I'm in favour.

wce needs to be preserved but cache wouldn't need to be AFAICT.

Regards,

Anthony Liguori


I think writeback is probably a better default than none.





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