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

[Jiri Denemark]

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