HAMMER update 10-Feb-2008

[Matthew Dillon]

:Matthew Dillon wrote:
:>     Here's what is left:
:>      * Structural locking.  The B-Tree is fine-grained locked but the
:>        locks for the blockmap are just a hack (one big lock).
:
:Have you decided how to implement multi-master replication yet?
:
:--
:Jason Smethers

    My current plan is to use a quorum algorithm similar to the one I wrote
    for the backplane database years ago.  But there are really two major
    (and very complex) pieces to the puzzle.  Not only do we need a 
    quorum algorithm, but we need a distributed cache coherency algorithm
    as well.  With those two pieces individual machines will be able
    to proactively cache filesystem data and guarantee transactional
    consistency across the cluster.

    The quorum algorithm is fairly straightforward, all the complexity there
    is basically on how to deal with broken connections, missing hosts,
    and things of that ilk.

    The caching algorithm is going to be a lot more complex.  It will have
    to have timeouts with quorum-based watchdogs for refreshment in order
    to deal with hosts that drop out of the cluster, on top of
    everything else it has to do.  It will have to be range-based at
    multiple levels in order to limit the memory footprint and work with
    super-large filesystems.

    I don't even want to start thinking about it yet.  What HAMMER will
    provide to the system as a whole is the real-time mirroring aspects,
    inode numbers and transaction id's which are forever-unique (allowing
    data to be passively cached indefinitely), historical access for
    as-of transactions which will allow parallel transactions to occur
    and detect collisions at commit time rather then during the transaction,
    and a bunch of other features.

					-Matt
					Matthew Dillon 
					<dillon at backplane.com>