Modular arrays earn new trust


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The bid sheet for the modular arrays slides across the conference room table, prompting eyebrows to rise. The storage manager looks the vendor squarely in the eye and says, "The price looks good, but here's the bottom line: We're betting our business on your modular arrays, so we've got to know, can we trust them?"

Increasingly, the answer is "yes." Modular arrays have come a long way in recent years. Analyst firms such as Gartner Inc., Stamford, CT, and The Evaluator Group, Greenwood Village, CO, now pin the enterprise-class label on modular arrays. Vendors promise the same or greater levels of availability, capacity, management and performance for their modular arrays as found on their monolithic arrays.

Toss in faster deployment, smaller footprints and lower price points and the decision would almost seem like a no-brainer, except for the critical environments into which users may look to deploy these arrays.

If users are to feel confident about moving their most critical data onto modular arrays in whatever environment they deploy them, they need to know the pros and cons of such a tactic. Management, security, performance and support along with price all factor into the equation. So let's look at the distinctions between modular and monolithic arrays, what features modular arrays now offer, how users should look to use them and what trade-offs, if any, users will encounter if they decide to go modular.


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of modular arrays
Modular arrays differ from monolithic arrays in four distinct ways:
  1. The flexibility to add disks
  2. Network connectivity
  3. How cache is managed on the controllers
  4. Support for different drive technologies
According to Ken Steinhardt, EMC Corp.'s director of technology analysis, "On modular arrays, you can add individual disk or processor enclosures on the fly, while monolithic arrays come with preconfigured slots for disks and processors."

All monolithic arrays offer either ESCON or FICON in addition to Fibre Channel (FC) or SCSI. All modular arrays--with the exception of EMC's DMX 800--only offer FC, SCSI and iSCSI as connectivity options.

Randy Kerns, partner at The Evaluator Group, says monolithic and modular arrays also handle caching differently. Monolithic arrays hearken back to their mainframe roots and keep all read and write cache as a single image; all reads and writes must be mirrored between the caches on the separate controllers before a response is sent back to the server. Modular arrays maintain writes as a single image before sending a response back to the server; their read cache is specific to each controller. Each controller handles reads independently. Responses are potentially faster because there's no requirement to mirror the read cache.

The final variation comes in the type of internal disk drives supported by the storage arrays. Monolithic arrays exclusively support either internal FC or SCSI disk drives. Modular arrays support those as well, but some vendors have started to add Serial ATA (SATA) or ATA drives to their stable of drive offerings to handle the growing need to store reference and backup data.

Already, products such as EMC's Clariion and Centera lines, EqualLogic's PeerStorage Array 100E, IBM's FAStT900 and Winchester Systems' AT-1200 support this sort of functionality. Hitachi Data Systems (HDS) is scheduled to ship these drives in the first quarter of 2004 for its 9500 line of modular arrays. While more storage vendors now offer either ATA and SATA drives along with SCSI or FC drives in their line of monolithic arrays, the ability to mix and match them in the same array still only shows up on the drawing boards.

Other than these specific differences, Kerns says, "When looking at functionality, reliability and performance, modular storage arrays are now virtually indistinguishable from monolithic storage arrays. In fact, they are just as reliable, easier to manage and typically outperform the cache centric boxes for the number of controllers they offer."

Users can now confidently deploy modular arrays in production for essentially any type of application previously reserved only for monolithic arrays. Both hardware and software features once only found in monolithic arrays now reside in modular arrays.

On the hardware side, modular arrays come with redundant controllers, increasing amounts of cache, multiple FC ports and high disk capacity. HDS' Thunder 9580V and EMC's DMX 800 both illustrate these sorts of advances. The 9580V provides two controllers, up to 8GB of cached memory, eight front-end FC ports and 64TB of raw capacity. Similarly, EMC's DMX 800 provides up to 32GB of global memory, 16 front-end FC ports and 17.5TB of raw capacity, while being one of the few storage array vendors to offer iSCSI capabilities. (See "Select modular storage arrays")

On the software side, users should look for features such as point-in-time copy, remote copy, LUN masking and easy-to-use management consoles. Both IBM's FAStT900 and FAStT700 storage servers offer Remote Copy, FlashCopy and VolumeCopy as optional features. Network Appliance Inc.'s (NetApp) FAS200 has SnapMirror, SnapRestore and SyncMirror options. You can create point-in-time copies at either local or remote sites for backup and recovery or development testing with copies of production data.

This was first published in January 2004

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