Compellent Storage Center 4.0

  • Editors' rating
    8.5 Excellent


  • Active data is automatically migrated to the fastest disks and disk tracks
  • Little-used files are migrated to the cheapest drives
  • Thin provisioning and thin import minimises the number of disks required
  • Quick and easy to manage


  • No facilities to eliminate duplicate storage blocks

Here's a fact worth knowing: very little of the data held on expensive Storage Area Networks (SANs) is ever accessed. Furthermore, companies routinely spend huge amounts providing enough space to cope with growth — much of which will never, ever, be used. Compellent understands this dilemma, and its Storage Center solution leverages the company's unique block-level virtualisation technology to neatly address both these and other common storage issues.

The Compellent hardware
Storage Center is a complete SAN solution that's based on industry-standard hardware — in contrast to the proprietary technology employed by some other SAN vendors. To this end, the Compellent storage controller is effectively an Intel-based server, running a custom OS booted from flash memory. Redundant power and cooling naturally come as standard, as you would expect on this kind of product, while a second controller can be added to provide automatic failover in the event of a hardware failure.

The Compellent solution is based on industry-standard server and SAN technology.

Because it is standards-based, the storage controller is easy to configure and expand, with a choice of Fibre Chanel and/or iSCSI connectivity using plug-in adapters. Up to 18 host ports can be configured as and when they're required; there's also full support for all the leading networks and server platforms, with no need for special agents.

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Tools are included to monitor and manage the industry-standard hardware on which the Compellent SAN is based.

There's flexibility at the back end too, with the storage controller connected to one or more disk enclosures — again, via plug-in Fibre Channel adapters. Up to 16 disks can be fitted per enclosure and you can mix together high-speed Fibre Channel and cheaper but slower SATA drives. Up to 1,008 disks can be accommodated in all, giving a maximum raw capacity of 504TB using currently available products.

The drive enclosures also come with redundant power; disks are also hot-swappable with RAID support, naturally, built in. Storage held on NAS servers can also be included as part of the Compellent SAN.

The Compellent software
The hardware, then, is nothing to get excited about. Fortunately, the software and associated technologies employed to manage the SAN are anything but run-of-the-mill.

One of the key enabling technologies here is Compellent's Dynamic Block Architecture (DBA), a storage virtualisation technology whereby every block of data held on the SAN is tracked independently. To do this, the software maintains tables of metadata detailing, for example, when each block was written, when it was last accessed, the frequency of access, the volume involved, disk location, RAID level and so on. As a result, data blocks can be located anywhere on the SAN, regardless of the logical volume with which they're associated, and quickly moved around as required. This enables the Compellent SAN to deliver a number of unique features.

Inactive data is automatically migrated to cheaper SATA disks, while the most heavily used blocks are kept on the fastest tier of Fibre Channel drives.

One of the most significant is Automated Tiered Storage, whereby data is intelligently migrated within the storage array based on how it's used. The most actively used blocks, for example, will be placed onto the fastest-spinning Fibre Channel disks, while those associated with inactive files get moved down to the slower, cheaper SATA drives. Given that up to 80 per cent of stored data is never actually used, this helps to save money while at the same time improving overall performance. Data can even be optimised at the drive level, with the most active blocks positioned on the fastest outer tracks of each disk, further enhancing retrieval speed.

Compellent's Dynamic Block Architecture also does away with the need to pre-allocate space when creating volumes: instead, DBA allocates blocks on the fly as they are needed — a feature Compellent calls Thin Provisioning. This makes it possible to create volumes that are much larger than the amount of free space available, reducing the number of disks required, while at the same time minimising power and cooling requirements.

With the latest 4.0 release you can also strip out unused space when migrating existing data onto the SAN, thanks to a feature called Thin Import. This can reduce storage overheads by up to 50 per cent. Of course, you need to know when space is really running low, so the software continually monitors what's being used then issues alerts to inform the administrator when more drives need to be added.

On a product of this complexity, professional installation and setup is to be advised and some training required. However, we found the Windows- and web-based management interfaces very intuitive; Automated Tiered Storage and other automatically handled features also leave little for the operator to do. Of course, you still have to configure the volumes you need and manage free space, but that isn't difficult: you can create new volumes in just a few seconds. We didn't encounter any issues when taking snapshots either, with full support for Windows VSS available to make it easier to back up running systems.

Comprehensive monitoring and reporting tools are included as part of the Storage Center solution.

Comprehensive monitoring and reporting tools are available, along with optional facilities to recover space allocated to deleted files on Windows servers and to optimise storage settings to suit different applications.

The overall result is a very flexible industry-standard-based SAN, with a number of unique features that can be used minimise the investment required while delivering a powerful and highly scalable storage solution.