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

[Dietmar Maurer]

> Maybe the best approach is to maintain a dirty bitmap while the guest is
> running, which is fairly cheap.  Then you can use the dirty bitmap to only 
> hash
> modified clusters when building the Merkle tree - this avoids reading the
> entire disk image.

Yes, this is an good optimization.

> > > I still think it's a cool idea.  Making it work well will require a
> > > lot more effort than a dirty bitmap.
> >
> > How do you re-generate a dirty bitmap after a server crash?
> 
> The dirty bitmap is invalid after crash.  A full backup is required, all 
> clusters
> are considered dirty.
> 
> The simplest way to implement this is to mark the persistent bitmap "invalid"
> upon the first guest write.  When QEMU is terminated cleanly, flush all dirty
> bitmap updates to disk and then mark the file "valid"
> again.  If QEMU finds the file is "invalid" on startup, start from scratch.

Or you can compared the hash keys in that case?

Although I guess computing all those SHA1 checksums needs a considerable amount 
of CPU time.