Am 29.06.2011 15:00, schrieb Avi Kivity: > 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.
Means that we need to move the WCE flag into guest state first, but that's what Anthony suggested, too. So I think we can do that. > I think writeback is probably a better default than none. Alex had a good point that O_DIRECT doesn't work everywhere, so I agree. Kevin
