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@xxxxxxxxxx - info@xxxxxxxxxx
GPG public key ID: FF5F32A8
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