libvirt users - Feb 2017 - Unsafe migration with copy-storage-all (non shared storage) and writeback cache

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@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@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8

On Fri, Feb 17, 2017 at 12:33:28 +0100, Gionatan Danti
wrote:
> 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.
I think your understanding is correct.
> 1) it is safe to do a live migration with --copy-storage-all when guests 
> are using writeback cache mode?
Yeah storage migration should be safe regardless of the cache mode used.
> 2) if so, why libvirt complains about that?
This looks like a bug in libvirt. The code doesn't check whether an
affected disk is going to be migrated (--copy-storage-all) or accessed
on the shared storage.

Jirka

On 17/02/2017 19:50, Jiri Denemark wrote:
>
> Yeah storage migration should be safe regardless of the cache mode used.
>
Very good news, thanks. Let me be paranoid: is it *surely* safe?
>
> This looks like a bug in libvirt. The code doesn't check whether an
> affected disk is going to be migrated (--copy-storage-all) or accessed
> on the shared storage.
>
> Jirka
>
Any chances the bug will be corrected? Should I open a BZ against it?

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