libvirt users - Feb 2017 - Libvirt behavior when mixing io=native and cache=writeback

Hi all,
I write about libvirt inconsistent behavior when mixing io=native and 
cache=writeback. This post can be regarded as an extension, or 
clarification request, of BZ 1086704 
(https://bugzilla.redhat.com/show_bug.cgi?id=1086704)

On a fully upgraded CentOS6 x86-64 machine, starting a guest with 
io=native and cache=writeback is permitted: no errors are raised and the 
VM (qemu, really) silently use "io=threads" instead. A warning should
be
raised, but I find reasonable that the VM is permitted to run.

On a fully upgraded CentOS7 x86-64 machine, starting a guest with 
io=native and cache=writeback is *not* permitted: an error is raised and 
the guest is not started. While it is absolutely OK to alert the user 
about the configuration error, I feel the VM should be started as on 
CentOS6. Anyway, this is not a big problem when dealing with a single host.

A more concerning behavior is that when trying to migrate a guest from 
the very first CentOS6 machine, which starts and runs such a guest with 
io=native and cache=writeback without problems, to a CentOS7 host, live 
migration aborts with an error:

[root@source] virsh migrate Totem --live --copy-storage-all --persistent 
--verbose --unsafe qemu+ssh://root@172.31.255.11/system

error: unsupported configuration: native I/O needs either no disk cache 
or directsync cache mode, QEMU will fallback to aio=threads

This error persists even if the VM config file is changed to use 
"io=threads". Of course, the running image continue to have
"io=native"
on its parameters list, but it does not really honor the "io=native" 
directive, so I find strange that live migration can not proceed. I 
think it is very reasonable to raise an error, but such error should not 
block the live migration process.

In short: there is some method to live migrate a running VM configured 
with "io=native" (but which is really using "io=threads"
internally) and
"cache=writeback"?

Thanks.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8

On Fri, Feb 17, 2017 at 02:52:06PM +0100, Gionatan Danti
wrote:
>Hi all,
>I write about libvirt inconsistent behavior when mixing io=native and
>cache=writeback. This post can be regarded as an extension, or
>clarification request, of BZ 1086704
>(https://bugzilla.redhat.com/show_bug.cgi?id=1086704)
>
>On a fully upgraded CentOS6 x86-64 machine, starting a guest with
>io=native and cache=writeback is permitted: no errors are raised and the
>VM (qemu, really) silently use "io=threads" instead. A warning
should be
>raised, but I find reasonable that the VM is permitted to run.
>
>On a fully upgraded CentOS7 x86-64 machine, starting a guest with
>io=native and cache=writeback is *not* permitted: an error is raised and
>the guest is not started. While it is absolutely OK to alert the user
>about the configuration error, I feel the VM should be started as on
>CentOS6. Anyway, this is not a big problem when dealing with a single host.
>
>A more concerning behavior is that when trying to migrate a guest from
>the very first CentOS6 machine, which starts and runs such a guest with
>io=native and cache=writeback without problems, to a CentOS7 host, live
>migration aborts with an error:
>
>[root@source] virsh migrate Totem --live --copy-storage-all --persistent
>--verbose --unsafe qemu+ssh://root@172.31.255.11/system
>
How about using --xml to supply the destination XML which would have
io="threads" specified?
>error: unsupported configuration: native I/O needs either no disk cache
>or directsync cache mode, QEMU will fallback to aio=threads
>
>This error persists even if the VM config file is changed to use
>"io=threads". Of course, the running image continue to have
"io=native"
>on its parameters list, but it does not really honor the
"io=native"
>directive, so I find strange that live migration can not proceed. I
>think it is very reasonable to raise an error, but such error should not
>block the live migration process.
>
>In short: there is some method to live migrate a running VM configured
>with "io=native" (but which is really using "io=threads"
internally) and
>"cache=writeback"?
>
>Thanks.
>
>--
>Danti Gionatan
>Supporto Tecnico
>Assyoma S.r.l. - www.assyoma.it
>email: g.danti@assyoma.it - info@assyoma.it
>GPG public key ID: FF5F32A8
>
>_______________________________________________
>libvirt-users mailing list
>libvirt-users@redhat.com
>https://www.redhat.com/mailman/listinfo/libvirt-users

On 20/02/2017 14:02, Martin Kletzander wrote:
> On Fri, Feb 17, 2017 at 02:52:06PM +0100, Gionatan Danti wrote:
>
> How about using --xml to supply the destination XML which would have
> io="threads" specified?
>
This seems to work, excellent suggestion! Thank you.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8