Re: [Qemu-devel] Default cache mode

[Christoph Hellwig]

On Wed, Jun 29, 2011 at 04:11:53PM +0200, Kevin Wolf wrote:
> If the guest is acting correctly, then this assumption is definitely
> true with cache=none/writeback. What needs to happen is that the guest
> issues a disk flush before sending your "transaction ok" message. If it
> doesn't do that it can fail even on real hardware. cache=writethrough
> makes things safe that aren't even safe on real hardware, that's why I
> consider it extremely conservative.

Depends on your defintion of real hardware.  Traditionally disks always
had WCE=0 semantics.  When ATA disks added volatile writecaches and people
started seeing problems due to it the advertisement of cache modes and
cache flush commands were added.  For non-ATA disks like SAS or FC
WCE=1 still is the typical delivery setting.