Mnesia: Load sharing and redundancy
Tue Mar 9 14:13:33 CET 2004
Carlos> Hi, I want to implement load sharing without loosing redundancy.
Carlos> Can anyone give me an example on how to fragment and replicate a
Carlos> Mnesia database to have this implemented? I mean referencing, how
Carlos> many nodes, machines and fragments. And the distribution of them.
Carlos> For example I figure that this could be a good solution, isn't it?
Carlos> Two nodes in one machine and two other nodes in another machine. Nodes
Carlos> A and B in machine M1, nodes C and D in machine M2.
Carlos> All DB tables fragmented in two fragments one fragment resident at
Carlos> node A, and one fragment at node C (then we use M1 and M2 machines to
Carlos> have load sharing).
Carlos> This implementation replicated as follows: fragments at node A
Carlos> replicated at node D, and fragments at node C replicated at node B
Carlos> (then we have cross replicas - M1 fragments with M2 ones). See
Carlos> attachment for representation.
Carlos> I would apreciate your answer very much, to have my DB implemented as
Carlos> soon as possible.
Carlos> Thanks, Carlos E. Silva.-
In order to keep things simple, I would suggest that you:
- Only have one node per machine. Why have you choosen to have two
nodes per machine?
- Avoid addition of new fragments to existing tables while system is
in full operation. Addition of new fragments is quite expensive as
it will imply existing records to be rehashed and moved between
fragments. A plain move of a fragment is cheaper.
- Avoid huge fragments. If you expect large data volumes you should
distribute it over more fragments than you would do for small data
volumes. This would give you a better precision in your load
balancing and lesser disturbance when you move fragments between
- Try to estimate the maximum number of nodes (machines) that you
that you ever will use in your system. The number of initial
fragments should always be more than the maximum number of nodes,
perhaps a factor 5 or 10 more.
- Take a look at the mnesia/examples/bench example.
If your maximum configuration only is two nodes there are no point
in using fragmented tables at all, for the purpose of load balancing.
High Availability Software, DBMS Internals
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