Re: [PATCH v2 28/29] migration: Add direct-io parameter

[Daniel P . Berrangé]

On Tue, Oct 31, 2023 at 12:52:41PM -0300, Fabiano Rosas wrote:
> Daniel P. Berrangé <berrange@redhat.com> writes:
> >
> > I guess I'm not seeing the problem still.  A single FD is passed across
> > from libvirt, but QEMU is free to turn that into *many* FDs for its
> > internal use, using dup() and then setting O_DIRECT on as many/few of
> > the dup()d FDs as its wants to.
> 
> The problem is that duplicated FDs share the file status flags. If we
> set O_DIRECT on the multifd channels and the main thread happens to do
> an unaligned write with qemu_file_put* then the filesystem will fail
> that write.

Doh, I had forgotten that sharing.

Do we have any synchronization between multifd  channels and the main
thread ?  eg does the main thread wait for RAM sending completion
before carrying on writing other non-RAM data ?  If not, is it at all
practical to add such synchronization. IOW, to let us turn on O_DIRECT
at start of a RAM section and turn it off again afterwards.

With regards,
Daniel
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