Re: [PATCH v2 18/18] migration/ram: Add direct-io support to precopy file migration

[Peter Xu]

On Thu, May 23, 2024 at 04:05:48PM -0300, Fabiano Rosas wrote:
> We've recently added support for direct-io with multifd, which brings
> performance benefits, but creates a non-uniform user interface by
> coupling direct-io with the multifd capability. This means that users
> cannot keep the direct-io flag enabled while disabling multifd.
> 
> Libvirt in particular already has support for direct-io and parallel
> migration separately from each other, so it would be a regression to
> now require both options together. It's relatively simple for QEMU to
> add support for direct-io migration without multifd, so let's do this
> in order to keep both options decoupled.
> 
> We cannot simply enable the O_DIRECT flag, however, because not all IO
> performed by the migration thread satisfies the alignment requirements
> of O_DIRECT. There are many small read & writes that add headers and
> synchronization flags to the stream, which at the moment are required
> to always be present.
> 
> Fortunately, due to fixed-ram migration there is a discernible moment
> where only RAM pages are written to the migration file. Enable
> direct-io during that moment.
> 
> Signed-off-by: Fabiano Rosas <farosas@suse.de>

Is anyone going to consume this?  How's the performance?

It doesn't look super fast to me if we need to enable/disable dio in each
loop.. then it's a matter of whether we should bother, or would it be
easier that we simply require multifd when direct-io=on.

Thanks,

-- 
Peter Xu