Once hard-and-fast product categories, the lines that separate monolithic from modular storage are getting increasingly blurry. Not only are modular systems getting bigger, faster and more capable, but newer monolithic systems are taking their cues from more flexible, configurable midrange systems.

The term "monolithic" usually applies to systems like EMC's Symmetrix, Hitachi Data Systems' Lightning and IBM's DS8000. These are high-end systems that have traditionally required customers to buy all the infrastructure they'll need up front, including the I/O controllers, the cache and even the frames; the only thing you could buy over time was additional disk drives.

That doesn't sit well with users who like the idea of being able to buy performance and capacity as they go. "Our customers have told us that they want to match their initial purchase to what they need and grow it only as needed," says Craig Butler, manager of IBM's TotalStorage disk solutions. IBM responded to that request last fall with the introduction of the DS6000 system, a companion to the larger DS8000 that runs the same code, but comes in a 3U...

Barry Burke, EMC's senior director of Symmetrix platform marketing, concedes this lack of flexibility has also been a problem for Symmetrix DMX customers. With EMC's monolithic systems, "the customer had to figure out where he's going to be within the life of the product and grow into it," he says.

EMC's new Symmetrix DMX-3, announced last month, begins to address this point by allowing customers to start with six data directors, EMC's term for I/O controllers, and add another pair as needed. EMC will probably announce a DMX model in the future that can scale over time from four to eight data directors, Burke says.

DMX-3's modularity also extends to its physical packaging. Unlike previous generations, DMX-3 has been divided into separate system and drive bays. You can buy a system today with a single disk bay (360 disk drives) and add another bay later.

Expect to see more modularity creep into high-end systems, says Butler. "I can guarantee you that no one is working on how to make their systems more monolithic."