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Spotlight on midrange arrays

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IBM's "more choice" marketing plan
When IBM introduced the DS6800 last November, industry pundits said it spelled the beginning of the end for the company's FAStT midrange product line. IBM readily admits there's an overlap between the two products, but insists it's a deliberate strategy to offer customers more choices.

Some customers, however, are less convinced of the plan. "The DS6000 series will eat the FAStT for lunch when you look at the TCO ... I see no reason to buy the lower-end products now," says Bob Venable, manager of enterprise systems

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at BlueCross BlueShield of Tennessee in Chattanooga. Craig Butler, IBM brand manager for midrange storage products, says the DS4500 (formerly FAStT 900) suits different applications than the DS6800 and users should select a box according to their needs.

What are the differences? The DS4500 has a maximum capacity of 32.7TB, while the DS6800 maxes out at 74TB. The DS6800 comes with 16 drive bays, while the DS4500 comes with none and requires an expansion unit to provide 14 drive bays. The DS4500 supports SATA, but SATA drives will come shortly to the DS6800, Butler says. Both systems support 2Gb Fibre Channel. But the RAID options are different on each system--the DS6800 supports RAID 5 and RAID 10, whereas the DS4500 offers RAID 0, 1, 3, 5 and 10. In addition, the DS6800 attaches to IBM's iSeries and L-Series servers while the DS4500 doesn't.

"Users who have DS4000 gear, aren't ready to swap it out and require SATA for nearline storage purposes today or extra RAID levels, will buy the DS4500," says Butler.

Given that the code base for the DS6000 series is completely different from the DS4000 line's, it doesn't sound like it's a simple case of swapping out the products. The replication and copy services between the two lines are entirely different code. In fact, IBM uses a replication appliance from CNT between the two boxes with scripts to convert the data. Over time, it's likely that SAN Volume Controller will smooth over this migration process between the product lines. But for now, IBM insists the DS6800 is at the bottom of the enterprise class, while the DS4500 is top of the midrange class and that they suit different needs.

The devil appears to be in the details of the pricing of these two systems. List price for the DS6800 with 4GB of cache and no drives is $86,500; list price for the DS4500 with 2GB of cache and no drives is $66,500. But here's the real killer: Users pay a flat price for additional capacity on the DS4500, which IBM says is twice as high on a per-terabyte basis vs. the more flexible per-terabyte pricing on the DS6800.
--Jo Maitland

Volume management
Sophisticated volume management software on midrange arrays is approaching the functionality found with monolithic arrays. In addition, every midrange array comes with software that lets administrators monitor, analyze, manage or tune the performance of the array with varying levels of granularity. For example, the base module of StorageTek's SANtricity software suite lets users:

  • Update controller firmware non-disruptively
  • Migrate RAID levels dynamically
  • Add and configure new drive modules
  • Manage a system with mixed FC and SATA disks
  • Monitor and tune performance

It's important to determine how many arrays a particular vendor's software will manage. Not having to learn how to use different management programs for all the different arrays in the data center saves considerable time. HDS, for example, extends the same level of software support it offers on its arrays to other vendors' midrange arrays by licensing and rebranding AppIQ Inc.'s StorageAuthority Suite as its HiCommand Storage Services Manager.

For any midrange array that will present virtual volumes or logical unit numbers (LUNs) to multiple servers on the same front-end FC port, volume management software is a must. While every array vendor offers this functionality in some capacity, there are differing degrees of management flexibility. The following volume management features should be considered essential:

  • Virtualization--the ability to create virtual volumes out of the raw disk.
  • Volume creation--control of how the virtual volumes are created, including their initial size and which raw disks are used in their creation.
  • LUN security--the ability to control which servers can access a specific virtual volume.
  • Volume groups--the ability to take existing virtual volumes, group them logically as one entity and present this new logical entity to the host as one large, logical virtual volume.
  • Dynamic volume growth--the ability to dynamically increase the size of an existing virtual volume, whether it's a single virtual volume or a volume group.

The ability to group two or more volume groups and present them as one large volume makes the most sense for offloading volume management from the server to the array. Offloading the volume management to the array lets storage admins working with heterogeneous server OS environments learn only one volume management interface. It can also eliminate the need to buy third-party, server-level volume management software. Nearly every midrange array can accomplish the offloading of this task, but each array uses different methods; users need to be cautious about changing existing volume configurations.

For example, EMC's Navisphere Management Suite allows a user to create volume groups, or what EMC calls metaLUNs, on its Clariion array. These metaLUNs can be created in either a striped or concatenated format from existing LUNs. Each option presents benefits and drawbacks. The striped feature provides better performance because data is striped across all of the LUNs in the metaLUN, although all of the LUNs in a striped metaLUN must be of the same size, RAID level and disk type. A concatenated LUN must also be composed of disks of the same type (FC or SATA) and RAID level, but concatenation allows individual LUNs of different sizes to be joined.

In addition, the maximum size of a metaLUN is restricted by the size of the individual LUNs and the type of Clariion array. For example, metaLUNs on the CX600 can comprise up to 16 LUNs, while only eight LUNs can be used for metaLUNs on the CX400 and CX200 models.

Shops that opt to offload volume management from the server to the array will also need the ability to extend or grow these volume groups as they fill to capacity. Midrange arrays from 3PAR, HP, EMC and other vendors provide dynamic volume group growth, but each handles it differently. 3PAR allows administrators to grow a volume by increasing it to the precise size desired. HP's EVA can start a volume at any size, ranging from 1GB to 2TB, and then grow the volume in 1GB increments.

EMC permits the dynamic growth of metaLUNs, but the process differs depending on how the metaLUN was created. If a LUN is added to a metaLUN that was created in a striped manner, the Clariion will re-stripe the existing data across all of the LUNs now in the metaLUN. With a concatenated metaLUN, when a LUN is added to it, it gets appended to the end of the existing string of LUNs in the metaLUN with new data put on the new LUN in the group. With a concatenated metaLUN, data isn't automatically redistributed across this new configuration of LUNs.

The final step in growing virtual volumes is to configure the host server OS to discover the new size of the virtual volume. Some OSes such as Windows Server 2003 can do so dynamically, but users should exercise extreme caution by testing this functionality first and assuming any volume expansion will necessitate a reboot or, minimally, a rescan of the expanded volume to discover the additional capacity. Administrators should also check with the vendor to see how their OS handles the dynamic growth of volumes. Some array vendors report that data loss can occur if an OS doesn't recognize dynamic volume expansions.

Snaps and mirrors
Driven by SATA drives, shrinking backup windows and the need to create in-house disaster recovery procedures, array-based snapshot and mirroring are becoming common. Hibernia's Berthaut uses the synchronous mirroring capabilities of Xiotech's Magnitude Geo-Replication Services software between sites in New Orleans and Shreveport, LA, with a high degree of success.

Berthaut began using the synchronous mirroring feature as a stopgap measure because he was unsure how successful this approach would be due to the 700-mile roundtrip distance between the two sites.

"The Windows and Novell servers are extremely tolerant of the latency, but I'm still looking for a more acceptable asynchronous solution," says Berthaut. "I am currently evaluating Xiotech's TimeScale rapid restore appliance, an asynchronous mirroring product, and I plan to use it to replace the current synchronous mirroring process."

As midrange arrays take on more monolithic attributes, users should look to deploy midrange arrays for more mission-critical storage applications. They offer low-cost disk, high levels of performance and availability, easy to use software and effective replication technologies.

This was first published in March 2005

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