On Mon, 2010-05-10 at 15:13 -0400, Jeff Garzik wrote:
On 05/10/2010 02:26 PM, Jeff Darcy wrote:
> On 05/10/2010 11:38 AM, Steven Dake wrote:
>> As there is only one writer in every case, a simple lamport time stamp
>> would do the trick.
>
> How is there only one writer? Either there's only one possible writer
> globally, there's only one possible writer per object, or you have to
> coordinate/reconcile between possible writers - potentially across
> sites, despite partitions, etc. Nothing in the very brief description
If I understand the vinzvault project correctly, it will be used to
provide block-level storage to a node, somewhat similar to Amazon's
Elastic Block Storage (EBS).
For that case, each block device will likely have only a single writer:
the kernel/FUSE to which vinzvault is providing storage.
Certainly, one will have multiple writers to -different- storage areas,
but each individual vinzvault storage area would seem to be a
single-client scenario.
Ya thats correct. The one consistency issue I can see occurs during
migration + failure of a node where membership may change at the same
time a new node takes on writing activities. If this occurs, a read
which would have been targeted at position 3, may now be targeted at
position 2, while the write has been replicated to 4, 5, 6 but not 2
yet. In this case, that new node may end up reading an out of sync
replica. I had thought of using LTS for this since its very simple and
I am familiar with the algorithm. Maybe there are other choices. The
D1HT membership algorithm is not immediate and completes in Ologn time
so there is some opportunity for inconsistency in the membership on the
various nodes resulting in a migrated node reading from a different
replica then the main writing replica.
Thinking about migration and membership changes is worthwhile since it
presents the only real challenge I have been able to think up regarding
synchrony of replicas.
Regards
-steve
Jeff
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