Re: [Xen-devel] [PATCH] libxl: introduce an option for disabling the non-O_DIRECT workaround [and 1 more messages]

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

Ian Jackson,
reading the thread again, it looks like the original patch is correct as
it is. Few minor modifications were suggested but they are not really
required.

I think we should check it in as is now. Maybe even consider it for a
backport.

Are you going to take care of it?

Thanks,

Stefano

On Wed, 2 Apr 2014, Felipe Franciosi wrote:
> A little bump on this:
> 
> Yesterday I ran into a situation where I couldn't possibly get qemu to use 
> O_DIRECT when starting a VM through xl. Using O_DIRECT is obviously desirable 
> when comparing storage solutions (from disk configurations to virtualisation 
> data paths) as it allows control over the workload that is being sent to disk 
> (and other cases).
> 
> I can see this thread got abandoned when you reached the conclusion that you 
> couldn't have a flag/option that would ensure O_DIRECT is used (or not) in 
> view of some backends not supporting this configuration.
> 
> But does it really have to be a flag that ensures O_DIRECT is used?
> 
> I imagine we can have a "hint" flag instead. Something that hints the backend 
> to use O_DIRECT. If configurable, the backend will follow this setting. If 
> not, it will ignore it. For example, blkback (as it is) cannot cache 
> anything, so it always submits requests straight to the block layer as they 
> come. On the other hand, tapdisk could use O_DIRECT or not (it currently 
> always use it). Qemu already supports it (through the xenstore key 
> "direct-io-safe" in the Xen case).
> 
> Thoughts on this?
> 
> Cheers,
> Felipe
> 
> > -----Original Message-----
> > From: xen-devel-bounces@xxxxxxxxxxxxx [mailto:xen-devel-
> > bounces@xxxxxxxxxxxxx] On Behalf Of Ian Jackson
> > Sent: 28 November 2013 17:33
> > To: Konrad Rzeszutek Wilk; George Dunlap; xen-devel@xxxxxxxxxxxxxxxxxxxx;
> > Ian Campbell; Stefano Stabellini; alex@xxxxxxxxxxx
> > Subject: Re: [Xen-devel] [PATCH] libxl: introduce an option for disabling 
> > the
> > non-O_DIRECT workaround [and 1 more messages]
> > 
> > Ian Jackson writes ("Re: [Xen-devel] [PATCH] libxl: introduce an option for
> > disabling the non-O_DIRECT workaround [and 1 more messages]"):
> > > But maybe the tristate is a reasonable suggestion.
> > 
> > I remember why we didn't do this.  We could implement
> >   no-direct-io-safe
> > or something, but it wouldn't be always effective.  It would only work for 
> > the
> > bits of the system that we can prevent from doing O_DIRECT.
> > 
> > Ian.
> > 
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxx
> > http://lists.xen.org/xen-devel
> 

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