Re: [Xen-devel] [PATCH v11 20/27] Support colo mode for qemu disk

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

> Furthermore, I think coming up with a story for PV-aware guests (PVHVM,
> PV and PVH) is also non-trivial. For one the disk replication logic is

Pls keep in mind that PV guests can use QEMU qdisk if they wish.

This is more of 'phy' vs 'file' question.
> not implemented in PV block backend,  we're not sure how feasible to
> replicate thing in QEMU into kernel, but we're quite sure it is not
> going to be trivial technically and politically. The uncertainty is too

.. and I think doing it in the kernel is not a good idea. With PVH
coming as the initial domain - we will now have solved the @$@#@@
syscall bounce to hypervisor slowpath - which means that any libaio
operations QEMU does on a file - which ends up going to the kernel
with a syscall - will happen without incuring a huge context switch!

In other words the speed difference between qdisk and xen-blkback
will now be just the syscall + aio code + fs overhead.

With proper batching this can be made faster*.

*: I am ignoring the other issues such as grant unmap in PVH guests
and qdisk being quite spartan in features compared to xen-blkback.

> big to come up with a clear idea what it would look like.

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