Re: [Qemu-devel] Default cache mode

[Kevin Wolf]

Am 29.06.2011 16:12, schrieb 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.

Sure, I'm not saying that it definitely breaks on any hardware, just
that it can break.

Kevin