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Re: [Xen-devel] [PATCH 1/3] qemu-xen-traditional: use O_DIRECT to open disk images for IDE

On Thu, 29 Mar 2012, Joseph Glanville wrote:
> On 29 March 2012 22:05, Stefano Stabellini
> >> Yes, correctness is important. But in some cases, the user may put the 
> >> performance at the first place. For example, our QA team has many cases 
> >> which will boot many guest at same time. If using no-cache mode, they need 
> >> to spend more time to wait the case finished. And this is not they wanted.
> >> For KVM, the qemu argument allow user to determine which cache mode they 
> >> like. I think we need to follow it. How about to add an option in config 
> >> file to allow user to choose the cache mode and the default value can be 
> >> no-cache.
> >
> > I think this is a good argument: we could add a new cache parameter to
> > the disk line parser and pass it to QEMU.
> > However we still need to decide what is the right thing to do by
> > default.
> Enabling writeback caching by default I think is probably an ill-advised 
> choice.
> The default in QEMU and KVM is writethrough as I understand it for
> much the same reasons as those I noted above.
> I would suggest setting the default to writethrough as per KVM/QEMU
> defaults as this is probably the least surprising choice.

I don't think it would be wise to switch from WB to WT by default,
considering the incredible difference in performance between the two.
Also as from my other email in this thread, I don't think it is
actually necessary to use WT to enforce correctness in all cases but
QCOW2. Moreover BZ 572825 has been fixed in upstream QEMU so it only
affects qemu-xen-traditional now.

As a consequence I think the best default behaviour we can have for
qemu-xen-traditional is:

                    IDE         QDISK
RAW            |    WB    |     NOCACHE
QCOW/QCOW2     |    WT    |     NOCACHE

For qemu-xen:

                    IDE         QDISK
RAW            |    WB    |     NOCACHE
QCOW/QCOW2     |    WB    |     NOCACHE

We should document that the IDE interface for HVM guests doesn't have the
same level of safeguards than QDISK in case of sudden host power downs
(as it has always been since that patch in 2009).

> The addition of an a parameter to match the QEMU invocation
> (cache=<none>,<writeback>,<writethrough>) would be most welcome.


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