Midrange SANs with high-end features from Compellent, Xiotech

Multiple-tier storage, data migration jazz up these smaller systems, but ease of management sets them apart

As high-end storage systems come out with new features, those features tend to migrate down over time to systems intended for the smaller enterprise. Two midrange SANs, Compellent’s Storage Center 3.3 and Xiotech’s Magnitude 3D 3000e, are taking advantage of this, offering a wealth of features that as recently as six months ago were limited to much more expensive systems.

The list of these previously high-end capabilities includes multiple tiers of storage (15,000-rpm Fibre Channel, 10,000-rpm FC, SATA), migration of data from one tier to another, the capability to migrate data from a RAID10 FC volume to a RAID5 SATA volume without interrupting access to the data, and the capability to expand existing volumes without reformatting. Given the rapid growth of data in most enterprises and the order-of-magnitude difference between high-end SCSI or FC drives and SATA-based storage, these features -- especially multiple tiers of storage in one system -- give administrators much-needed flexibility.

The products are comparable in price and features, although Compellent has an edge in performance and maturity -- what takes a dozen steps with the Xiotech system can be accomplished in a couple of steps with the Compellent system. It’s not that the Xiotech system won’t do what the Compellent box will do; it’s just that it’s more complicated.

Compellent Storage Center 3.3

To test these systems, I attached them to Windows and Linux hosts, created volumes and mapped them to specific hosts, created replicas of the initial volumes, expanded the volumes beyond their initial size, migrated volumes from first- to second-tier storage, and ran IOmeter performance tests.

The Compellent Storage Center consists of a storage controller and one or more drive shelves. The drive shelf accommodates 16 SCSI U320 or ATA drives. Setting up the system was as simple as installing the drives in the drive shelf, powering on, and connecting to the browser-based interface.

Compellent’s administrative interface is clean and easy to navigate, showing a level of maturity that Xiotech can’t quite match. Creating volumes is straightforward, and all tasks you might need to perform are readily accessible and well documented through the help system. A topology view allows you to see a logical map of the SAN and to click and drag to connect servers to volumes; a folder view allows assigning management of specific server/volumes to specific users.

Many of the setup tasks are automated, so administrators who haven’t been specifically trained on storage administration will find them less daunting. Wizards make initial setup straightforward. Just as simple to perform are tasks such as setting up replication or data migration.

The automated data migration is an excellent example of Compellent’s streamlining work. There are three tiers of storage available: RAID10 on high-performance drives, RAID5 on high-performance drives and SATA drives. You can define a high-performance partition as tier one or two, and a SATA partition as tier three.

Next, if you have the optional Data Progression software installed, data is migrated from the highest-performance tier to the lowest during a period of 12 days. This migration happens automatically, based on whether the data has recently been accessed and how often. Data Progression can be enabled on a per-volume basis, but it is not based on additional rules -- it’s either on or off for the whole volume and uses as many tiers of storage as are available.

Thin provisioning allows you to create a virtual volume as large as the OS will allow, of which only a small part is actual disk space. For example, the initial actual size of a 2TB virtual disk could be 10GB, but you will be able to expand that size dynamically as you wish, up to the 2TB size, without changing anything on the server. You can even resize the partition to more than 2TB, as long as the OS supports it.

Snapshots create copies of data that can then be used to recover data or make backups to tape. Snapshots use single-instance at the block level, so if you make a snapshot, change a few files, and make another snapshot, only the blocks that have been changed are rewritten. This saves huge amounts of space and speeds up the snapshot process. The default template for replays is daily, weekly, and monthly, but it’s easy to set to any frequency you like.

In addition to snapshots, you can replicate a volume in real time (synchronously), at scheduled intervals (asynchronously), and semi-synchronously -- the system doesn’t wait for acknowledgement before sending the next packet. An auto-replay feature restarts an asynchronous transfer if it’s interrupted to ensure data integrity.

Compellent offers excellent reporting tools, which show topics such as trends of space in use, progression, and migration. To upgrade other SANs, you can copy to or from external disks and Compellent storage units, although there is currently no way to designate external disks as a fourth tier. The system supports Microsoft’s VSS (Volume Shadow Copy Service) for backups of open databases and Exchange system files.

All in all, Compellent Storage Center 3.3 has all of the bells and whistles available on much more expensive systems, at an affordable price, and with excellent performance and usability to boot.

Xiotech Magnitude 3D 3000e

The Magnitude 3D 3000e system is the smallest of several systems offered by Xiotech, and it has the benefit of a clear and easy migration path that will not interrupt access to the data stored on the system.

The system I received for testing included one FC bay with 16 146GB 10K U320 drives (SBOD), one SATA bay with 12 400GB SATA drives, two storage controllers, and one ICON (Intelligent CONtroller) management module.

Management is done via the ICON Web interface, along with a few tasks performed through the DCN (Dimensional Controller Nodes) storage controllers’ Web interfaces. Xiotech requires only one DCN but recommends a dual-path, dual-controller setup for redundancy; only one ICON is necessary, no matter how many DCN controllers there are.

As with Compellent, Magnitude’s wizards perform most configuration tasks. The ICON management interface has separate log-ins for each type of storage -- the high-performance bay has one log-in, and the SATA bay has another. This separation can complicate some tasks, such as migrating a volume from the FC shelf to the SATA shelf, because you have to change log-ins to perform some of the necessary tasks. The multiple log-ins do allow granular management, but the Compellent system does the same in a more straightforward fashion by giving the overall manager delegation of any volume without requiring you to switch log-ins.

Moving from one Magnitude storage tier to another, or from one RAID level to another, involves creating a second virtual disk, using the copy and mirror procedure to copy the first partition, and then breaking the mirror and attaching the second copy to the server in place of the first. This procedure is done fairly easily through the UI -- and without interrupting access to data. It is, however, a more involved process than with Compellent.

Expanding an existing virtual disk is fairly straightforward, as is setting up the snapshots and replication features. The Magnitude management interface is a multiple-tab affair; finding the necessary screen to configure any given feature by browsing through the tabs was usually easy.

Magnitude’s SAN map shows a physical map of the SAN, using a block diagram. It has excellent reporting tools that show environmental statistics, I/O by vdisk or RAID array, by drive shelf, or by physical or logical device. It also has a useful built-in Active Watch tool that will report any crossing of performance thresholds via e-mail or SNMP alarms.

Tracking Performance

I ran Intel IOmeter tests on both systems’ first- and second-tier storage volumes and found that Compellent Storage Center consistently ran about 20 percent faster than the Xiotech Magnitude system on both high-performance and SATA volumes. Tests were run from the same server, using the same HBA, with newly created volumes for each test. Coupled with the Compellent system’s higher total capacity -- it supports unlimited capacity, compared with Xiotech’s 19.2TB limit -- these performance numbers give Compellent a stronger leg to stand on.

That’s not to say that the Magnitude 3D 3000e System is a bad choice; it’s a well-engineered system that would look good in comparison to most other midtier SAN systems on the market. The Compellent system, however, is more mature and shows it. It offers automatic data migration, a cleaner, easier-to-use interface, and higher performance, all for about the same price per gigabyte.

InfoWorld Scorecard
Interoperability (10.0%)
Scalability (20.0%)
Value (10.0%)
Performance (20.0%)
Reliability (20.0%)
Management (20.0%)
Overall Score (100%)
Compellent Storage Center 3.3 8.0 9.0 8.0 9.0 8.0 9.0 8.6
Xiotech Magnitude 3D 3000e 8.0 8.0 7.0 8.0 8.0 8.0 7.9