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

[Alex Bligh <alex@xxxxxxxxxxx>]

--On 8 March 2013 10:17:12 +0000 George Dunlap <George.Dunlap@xxxxxxxxxxxxx> 
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.

--
Alex Bligh

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