Dear reader ,
Thought to shed some light on to this , since I am also a Ask Tom fan and a Oracle DB fan.
For couple years Oracle RAC was the only transparently Horizontally scalable database in the market.
The IBM Purescale doest have much technical documents out there yet , but I managed to figure out few things.
- they took some concepts our of the z-series mainframe
- Boxed it in a P series HW with AIX/Linux
- Of course there is always a Mainframe version
So as per my understanding IBM DB2 Purescale allows two (could be more )centralized locking node's to be present for HA reasons in their configurations. requires minimum one purescale node for the solution to work in Cluster mode.
The purescale node acts as a dedicated message box for all other nodes. It is constructed with low latency IB/RDMA interconnects.
What they have basically done is created a additional node in the middle dedicated to handle what Oracle RAC does in Cache Fusion. As per the currently available literature , It seems to be limited to IBM HW. The Clever point to recognize is that they centralized the Caching to a single dedicated node and made that node highly available , which dramatically reduce the inter node synchronization traffic. Where as in Oracle RAC everybody know everything (for great extent) ..but this cause's some traffic.
Where as Oracle RAC is generic solution , that allows us to create on many platforms. PureScale I believe will more or less fight at the level of Oracle Exadata not with Vanilla RAC.
On the HW side of the techniques, they have invested on infini band with RDMA and exposing the DB2 code to to the Cache sync via that. Couple of years back Oracle RAC also took steps towards improving interconnect speed by adopting RDA protocol over IB(UDP) on Linux. As per IBM they can do the scaling via this dedicated RDMA concepts by bypassing OS layer and other additional layers (interrupts ).
I am sure if there is a Market competition from IBM for Exadata , it will only be a temporary set back for Oracle. Since decades a go Oracle and Sun did some really good innovative feature like Solaris KAIO ..which made a difference when it came to IO load.
I am sure with up coming Solaris critical application API and some focus on RDMA based optimization will allow the Oracle to do the same much better ! or they might do the whole Cache - Fusion operation in HW / dedicated solely for cache coherency.
Another point is that most of these approaches were materialized some time back in Clustering world ..., looks like it is all coming back ..., instead of proprietory interfac ..looks like vendors are joining the IB Band wagon.
For your scenarios on what happens to a data block and how it gets transferred and how the cache fusion is keeping this information is explained very well in two very good books on the subject.
- Real Application cluster handbook - K Gopalakrishnan
- Pro Oracle Database 10g RAC on Linux: Installation, Administration, and Performance By Julian Dyke , Steve Shaw
Last but not least , there is no master instance or master node concept in RAC, some times one could get confused with terms like master instance ...which simply refers to a state of a RAC memory structure in a given time quantum under specific Cache coherence state.
If you are interested more on IBM DB2 PuresScale , you can also follow this blog on ,
http://it.toolbox.com/blogs/db2luw/db2-purescale-scalability-part-1-35173 Thanks and Good luck ! Nothing can beat the Oracle Innovation !