Re: [Qemu-devel] [RFC 0/8] block: Live backup prototype

[Stefan Hajnoczi]

On Mon, Mar 11, 2013 at 03:00:38PM +0000, Dietmar Maurer wrote:
> > > I can see space reduction up to 4% using this feature. Considering the
> > > fact that it comes at no cost, it would be stupid to remove it.
> > 
> > Okay, looks like it's useful but not a huge win.
> > 
> > In the NBD approach pipelining writes (or discards) ought to make 4 KB zero
> > blocks usable without overhead.
> 
> But you need to re-asemble 64KB block from that - I guess that is the 
> difficult part here?

That's not hard.  It can be added to the RFC code I posted.  Here is
an outline.

Say there is a 64 KB block with 2 zero regions:

0123456789abcdef
XXXXzzXXXXXzXXXX

X - populated 4 KB block
z - zero 4 KB block

Send 3 writes:
NBD_CMD_WRITE from=0 length=16 KB
...
NBD_CMD_WRITE from=24 KB length=20 KB
...
NBD_CMD_WRITE from=48 KB length=16 KB
...

The NBD server can pack this back into an extent's blockinfo:

def add_cluster(cluster, blockinfo, blocks):
    ...the code to add a cluster to the current extent...

def prepare_for_blockinfo_update(cluster):
    # We're still filling in the same cluster, all is fine
    if cur_cluster == cluster:
        return

    # Flush out last cluster, which is now complete
    add_cluster(cur_cluster, cur_blockinfo, cur_blocks)

    # Prepare to work on a new cluster
    cur_cluster = cluster
    cur_blockinfo = dev_id << 32 | cluster
    cur_blocks = []

def write(from, size, data):
    cluster = from / VMA_CLUSTER_SIZE
    prepare_for_blockinfo_update(cluster)

    cur_blockinfo |= build_blockinfo_mask(from, size) # bit manipulation
    cur_blocks.append(data)

In other words, the NBD server assembles the writes into a blockinfo and
list of blocks.

Stefan