Re: [Qemu-devel] [PATCH v3 0/8] block: drive-backup live backup command

[Dietmar Maurer]

> >> True, but that would happen only in case the host crashes.  Even for
> >> a QEMU crash the changes would be safe, I think.  They would be
> >> written back when the persistent dirty bitmap's mmap() area is
> >> unmapped, during process exit.
> >
> > I'd err on the side of caution, mark the persistent dirty bitmap while
> > QEMU is running.  Discard the file if there was a power failure.
> 
> Agreed.  Though this is something that management must do manually, isn't it?
> QEMU cannot distinguish a SIGKILL from a power failure, while management
> can afford treating SIGKILL as a power failure.
> 
> > It really depends what the dirty bitmap users are doing.  It could be
> > okay to have a tiny chance of missing a modification but it might not.

I just want to mention that there is another way to do incremental backups. 
Instead
of using a dirty bitmap, you can compare the content, usually using a digest 
(SHA1) on clusters.

That way you can also implement async replication to a remote site (like MS do).