[libvirt-users] Unsafe migration with copy-storage-all (non shared storage) and writeback cache

[Gionatan Danti]

Hi list,
I would like to understand if, and why, the --unsafe flag is needed when 
using --copy-storage-all when migrating guests which uses writeback 
cache mode.

Background: I want to live migrate guests with writeback cache from host 
A to host B and these hosts only have local storage (ie: no shared 
storage at all).

 From my understanding, --unsafe should be only required when migrating 
writeback-enabled guests between two hosts which share non-cluster-aware 
storage (ie: NFS), but it should not be necessary when not sharing storage.

However, when trying to migrate from a CentOS6 host to a CentOS7 machine 
with command

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

I have the following error:
error: Unsafe migration: Migration may lead to data corruption if disks 
use cache != none

So, my questions are:

1) it is safe to do a live migration with --copy-storage-all when guests 
are using writeback cache mode?

2) if so, why libvirt complains about that?

3) if not, what is the best method to migrate a running quest using 
writeback cache mode?

Thanks.

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