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.
