Re: [PATCH v2 0/3] Add support for NetBSD gntdev

[Roger Pau Monné <roger.pau%citrix.com@localhost>]

On 30/11/12 09:52, Manuel Bouyer wrote:
> On Fri, Nov 30, 2012 at 09:42:52AM +0100, Roger Pau Monné wrote:
>> On 29/11/12 19:56, Manuel Bouyer wrote:
>>> On Thu, Nov 29, 2012 at 06:31:45PM +0100, Roger Pau Monne wrote:
>>>> The following series adds support for NetBSD gntdev to libxc, and 
>>>> makes libxl use Qemu as a disk backend if an image file stored on a 
>>>> remote filesystem is used.
>>>
>>>
>>> Can't you let the administrator decide this in the domU's config file ?
>>> Right now vnd on nfs has problems, but nothing unfixable.
>>> So one day you'll want to use the kernel driver for nfs too.
>>> But maybe other local filesystems will have the problem.
>>> So it's not for the tool do decide in the back of the admin.
>>
>> We can always add a configure check later when this is fixed to decide
>> if it is needed or not. I don't like the idea of having to set a
>> configuration option if you want to use disk images on NFS, because if
>> you don't know about this specific issue your whole Dom0 will crash
>> which is really not desired or user friendly.
> 
> And I don't like the idea of software doing things in my back.
> And, beside this, I don't think local vs remote is the right criteria.
> There are remote filesystems which may play nice with vnd. There
> are local filesystems that may not play nice with vnd.

I would agree with you if this was a DomU crash, but in this case the
crash happens in the Dom0, and every DomU that the system might be
running crashes completely. This is not acceptable in any way from my
point of view.

I think we should not expect the user to be aware of this kind of
problems. If we cannot guarantee that the vnd driver is functional for
all filesystems, we should not use it. From my point of view reliability
should always come before performance.