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Date: Wed, 29 Jun 2011 13:59:21 +0200
From: Kevin Wolf <kwolf@redhat.com>
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Subject: [Qemu-devel] Default cache mode
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To: Qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, Christoph Hellwig <hch@lst.de>, Anthony Liguori <anthony@codemonkey.ws>, Avi Kivity <avi@redhat.com>

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