[libvirt-users] Why librbd disallow VM live migration if the disk cache mode is not none or directsync

[Ming-Hung Tsai]

Michal Privoznik <mprivozn at redhat.com> :
>
> On 7/29/19 3:51 AM, Ming-Hung Tsai wrote:
> > I'm curious that why librbd sets this limitation? The rule first
> > appeared in librbd.git commit d57485f73ab. Theoretically, a
> > write-through cache is also safe for VM migration, if the cache
> > implementation guarantees that cache invalidation and disk write are
> > synchronous operations.
> >
> > For example, I'm using Ceph RBD images as VM storage backend. The Ceph
> > librbd supports synchronous write-through cache, by setting
> > rbd_cache_max_dirty to zero, and setting
> > rbd_cache_block_writes_upfront to true, thus it would be safe for VM
> > migration. Is that true? Any suggestion would be appreciated. Thanks.
>
> The commit you refer to is very old and my hunch is that things looked
> different in 2012. Things might have changed since then and if
> write-through wasn't safe ~7 years ago, it might be safe now (with some
> tuning).
>
> Michal

Yes, but now the limitation is still there, in
qemuMigrationSrcIsSafe(). Should we relax the limitation?


Ming-Hung Tsai