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

[Stefan Hajnoczi <stefanha@redhat.com>]

On Mon, Sep 02, 2013 at 02:57:23PM +0200, Benoît Canet wrote:
> 
> I don't see the point of using hashes.
> Using hashes means that at least one extra read will be done on the target to
> compute the candidate target hash.
> It's bad for a cloud provider where IOs count is a huge cost.
> 
> Another structure to replace a bitmap (smaller on the canonical case) would be
> a block table as described in the Hystor paper:
> www.cse.ohio-state.edu/~fchen/paper/papers/ics11.pdf

This is similar to syncing image formats that use a revision number for
each cluster instead of a hash.

The problem with counters is overflow.  In the case of Hystor it is not
necessary to preserve exact counts.  A dirty bitmap must mark a block
dirty if it has been modified, otherwise there is a risk of data loss.

A bit more than just counters are necessary to implement a persistent
dirty bitmap, but maybe it's possible with some additional state.

Stefan