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Message Posted: Tue, 02 May 2000 @ 08:36:35 GMT

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Subj:   Re: Scalability of External Storage
From:   David Wellman

Hi David,

There are a few which immediately come to mind.

Firstly, Teradata scalability is based around increasing the amount of computing resources in equivalent amounts compared to what you already have. In this way, your processing time can be maintained as you increase the amount of work. In hardware terms, this typically means that if you've currently got a machine with 'n' Nodes each with a certain amount of memory, cpu and external disk space then your system expansion path is to add more nodes, each with the same amount of resources (memory, cpu and external disk space). The numbers get slightly confused when you start adding nodes of different machine types - co-existence - but the principal is the same.

Secondly, there are finite limits for most things in computing and one of them is the amount of external disk space that NCR will support attached to any single Node. These limits are often placed there for very good reason - usually performance.

Thirdly, I note that you say that you're being told it's "impractical to add disk storage" rather than "it can't be done". Assuming that you wouldn't be going over any limits referred to above, then that comment is very accurate. Adding disk space to an existing configuration without adding extra nodes is not a straight-forward expansion in the Teradata world, it can mean an unload/reload of the entire system (which compared to the ease of Reconfig is a real pain).

Even if you just add external disk storage (no extra cpu's, memory etc), it is probable that your performance will start to suffer as you add more data. This is simply due to each node having to process more data, it just takes longer. That may be acceptable to you, but a lot of sites would not accept it.

The poorer performance is not just simply related to amount of data though. If you add more AMP's to handle the new disk space then these take more memory, which means less memory available for Teradata buffering (FSG cache) which is usually detrimental to the performance of any dbms. More Amps also means more cpu resources consumed simply managing them.

So, can you "just add external disk space"? Yes you can. It is not always an straight-forward expansion to do and may well result in worse performance of the system under some workloads.

I hope that's useful.


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