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. Kevin
