NFS and unsafe migration

[Gionatan Danti]

Hi all,
I have a question about NFS datastore and unsafe migration.

When migrating a virtual machine having a virtio disk in writeback cache 
between two hosts sharing a single NFS datastore, I get the following error:

"Unsafe migration: Migration may lead to data corruption if disks use 
cache != none or cache != directsync"

I understand why libvirt alerts for unsafe migration in cases where no 
coherency is enfoced by the underlying system; however, is it really the 
case for nfs?

 From what I know (and from the man page), by default nfs has 
open-to-close consistency, which seems quite right for migrating a 
virtual machine between (as only one host at a time reads/writes/locks 
the virtual disk files).

I know that I can simply use cache=none to let the problem go away; 
however, this significantly impairs performance when used on nfs.

Am I missing something?
Thanks.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti at assyoma.it - info at assyoma.it
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