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

[Stefan Hajnoczi]

On Wed, May 22, 2013 at 03:34:18PM +0000, Dietmar Maurer wrote:
> > That sounds like more work than a persistent dirty bitmap.  The advantage 
> > is that
> > while dirty bitmaps are consumed by a single user, the Merkle tree can be 
> > used
> > to sync up any number of replicas.
> 
> I also consider it safer, because you make sure the data exists (using hash 
> keys like SHA1).
> 
> I am unsure how you can check if a dirty bitmap contains errors, or is out of 
> date?
> 
> Also, you can compare arbitrary Merkle trees, whereas a dirty bitmap is 
> always related to a single image.
> (consider the user remove the latest backup from the backup target). 

One disadvantage of Merkle trees is that the client becomes stateful -
the client needs to store its own Merkle tree and this requires fancier
client-side code.

It is also more expensive to update hashes than a dirty bitmap.  Not
because you need to hash data but because a small write (e.g. 1 sector)
requires that you read the surrounding sectors to recompute a hash for
the cluster.  Therefore you can expect worse guest I/O performance than
with a dirty bitmap.

I still think it's a cool idea.  Making it work well will require a lot
more effort than a dirty bitmap.

Stefan