On 10/4/2012 12:19 PM, Richard Elling wrote:
On Oct 4, 2012, at 9:07 AM, Dan Swartzendruber <dswa...@druber.com <mailto:dswa...@druber.com>> wrote:


On 10/4/2012 11:48 AM, Richard Elling wrote:
On Oct 4, 2012, at 8:35 AM, Dan Swartzendruber <dswa...@druber.com <mailto:dswa...@druber.com>> wrote:


This whole thread has been fascinating. I really wish we (OI) had the two following things that freebsd supports:

1. HAST - provides a block-level driver that mirrors a local disk to a network "disk" presenting the result as a block device using the GEOM API.

This is called AVS in the Solaris world.

In general, these systems suffer from a fatal design flaw: the authoritative view of the data is not also responsible for the replication. In other words, you can provide coherency but not consistency. Both are required to provide a single view of the data.


Sorry to be dense here, but I'm not getting how this is a cluster setup, or what your point wrt authoritative vs replication meant. In the scenario I was looking at, one host is providing access to clients - on the backup host, no services are provided at all. The master node does mirrored writes to the local disk and the network disk. The mirrored write does not return until the backup host confirms the data is safely written to disk. If a failover event occurs, there should not be any writes the client has been told completed that was not completed to both sides. The master node stops responding to the virtual IP, and the backup starts responding to it. Any pending NFS writes will presumably be retried by the client, and the new master node has completely up to date data on disk to respond with. Maybe I am focusing too narrowly here, but in the case I am looking at, there is only a single node which is active at any time, and it is responsible for replication and access by clients, so I don't see the failure modes you allude to. Maybe I need to shell out for that book :)
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