Re: [PATCH] QEMU(upstream): Disable xen's use of O_DIRECT by default as it results in crashes.

[George Dunlap <George.Dunlap@eu.citrix.com>]

On Fri, Mar 8, 2013 at 10:27 AM, Alex Bligh <alex@alex.org.uk> wrote:
>
>
> --On 8 March 2013 10:17:12 +0000 George Dunlap <George.Dunlap@eu.citrix.com>
> wrote:
>
>> Wait, aren't O_DIRECT and BDRV_O_NOCACHE required for safety?  That
>> is, without these flags isn't it possible that the guest OS thinks
>> that the data has made it onto stable storage, while in fact it's
>> still in dom0's memory?  Or am I missing something?
>
>
> That's why Konrad asked me to do the blktrace and check barrier writes
> were getting through.
>
> In any case, the argument about safety seems slightly off base when we've
> already shown that even with O_DIRECT it's already returning before
> the write is actually done (on any form of network based storage), and
> moreover can crash dom0 and hence all the guests. If you have network
> based storage, not using O_DIRECT is infinitely safer.
>
> I suppose arguably it would be better if this was a configuration knob.

Sorry, obviously I wasn't following that thread.

If you can verify that it's always safe to use qemu without O_DIRECT
(or that it's not any safer to use it), then you should just remove
it; no point in having options which have no practical effect.  But in
that case you should make sure to mention in the commit message why
O_DIRECT isn't actually needed.

 -George