Re: xbd0: WARNING: cache flush not supported by backend

[Christoph Egger <Christoph_Egger%gmx.de@localhost>]

On 25.09.10 00:22, Manuel Bouyer wrote:
> On Fri, Sep 24, 2010 at 05:39:56PM -0400, Thor Lancelot Simon wrote:
>> On Fri, Sep 24, 2010 at 07:58:08PM +0200, Manuel Bouyer wrote:
>>>
>>> I guess this is because you're using -o log on the filesystems ?
>>> In order to work reliably, NetBSD needs to be able to flush the underlying's
>>> disk cache. This needs support in the backed driver, but this was not
>>> provisionned by the Xen peoples (I'm not sure how or if linux manages
>>> to run reliably a journaled filesystem without it) so it's a NetBSD
>>> extention to the Xen block device protocol.
>>
>> I have wondered about this.  Linux guests hosted on NetBSD dom0 complain
>> that they cannot do flushes or write barriers.  Hosted on Linux Dom0,
>> they work fine.
>>
>> So I have to conclude that we just implement cache flushes in a different
>> way than they do, not that they don't implement them.  Why?
> 
> I guess this is something that showed up in a later version of Xen then.

That's true. Sun (back then it existed) needed a cache flush feature for
Solaris. So they spoke up on xen-devel with a proposal. Solaris is
the first and only one known OS by the xen people that need it.

> When I wrote the cache flush

You did? When?

> I'm almost sure nothing related was present in the public interface.

Why didn't you speak up on xen-devel? We have a propietary interface
from the beginning because of that.

Do we also have an extension to handle 64k block accesses?
When noone speaks up on xen-devel then that means to the xen people,
noone needs it or does so.

Christoph