ILM (information lifecycle management) may be one of the latest crazes in the storage industry but many businesses are struggling with how it should be implemented. Compellent puts an interesting new twist on ILM as its Storage Center aims to provide a solution that removes all the hassle of data classification. Furthermore, it supports both fibre channel (FC) and iSCSI target presentation and delivers an impressive range of storage virtualisation technologies.

The Storage Center comprises multiple disk arrays that support high performance FC and low cost Serial ATA (SATA) hard disks and each array has room for up to sixteen drives. These come with dual, quad-port FC modules (the SATA arrays only use two ports on each module) and dual power supplies and all are hot-swappable.

For this review we were supplied with two arrays each with eight FC and SATA hard disks installed in each. At the business end is a well specified Supermicro 3U rack chassis and motherboard combo equipped with a 3.2GHz Xeon processor and 2GB of memory. Ignore the hot-swap drive bays at the front as the Linux-based kernel runs on an internal IDE CompactFlash card. The reason Compellent chose this system is the number of PCI slots on offer as these can all be populated with either FC controllers or iSCSI HBAs with TOEs (TCP offload engines). The review system came equipped for FC operations and was supplied with a pair of 2Gbit/s QLogic quad-port HBAs.

One of the key concepts behind the Storage Center is Compellent’s DBA (dynamic block architecture) technology which uses metadata to keep track of all blocks and delivers a unique range of features. It makes its presence felt when the system is installed as you first need to create Disk Folders and these can comprise all hard disks or some and can be a mixture of both. If you wish you can elect to use SATA drives for some Disk Folders and FC for others to provide low cost or high performance storage. Within each Disk Folder you create your virtual volumes and these can be assigned to specific FC and iSCSI hosts. During initial installation you configure your FC controllers and provide IP addresses to your iSCSI HBAs. With the hosts logged on to the controller you can view them and map volumes to those selected.

Compellent’s data progression feature is designed to automatically migrate data from FC to SATA storage as it becomes less frequently used. It creates up to three tiers of storage and a very slick feature is its ability to identify and categorise physical hard disks based on their interfaces and spindle speeds. For example, if you had 15K FC drives these would be placed in Tier 1, 10K FC drives would go into Tier 2 and SATA drives would make up Tier 3. The beauty of this is you don’t have to do this yourself and during volume creation you can select multiple RAID levels for each tier. As data is used less frequently it is migrated down through different RAID levels within each tier and then down to the next tier eventually arriving at Tier 3.

A critical factor here is that with DBA on the case all data progression is at the block level and not the file level. Only two settings are used to determine progression for each volume where you decide how many days a block has to remain unused for before it is moved and a ratio that controls how data is moved up and down the tiers. The latter is important as data that suddenly becomes popular will be automatically moved back up through the tiers and the ratio decides how quickly it occurs.

Most storage systems offer snapshots as standard but in the world of Compellent these become replays which use far less physical space. When volume snapshots are created they usually require a percentage of the volume to be stored in and will stop running when this space is used up. Replays use DBA to remember where blocks were when the replay was taken and only use up space as new data is written to the volume. When a replay is taken it defaults to using low cost Tier 3 storage but you can choose a higher level and allow data to be progressed down.

Support for dynamic capacity means you can purchase less storage than you think you need in complete safety. When a virtual volume is created it can be much larger than the physical size of the combined disks. Virtual volumes in MB, TB or even PB can be created and DBA simply keeps an eye on the physical amount of space being used and dynamically allocates disk blocks as required. When used space reaches eighty per cent the controller can warn you via the web interface, send multiple emails out and even advise Compellent support staff. The system also goes into a conservation mode where it turns off the write cache and will start expiring replays. You do need to react to these warnings as when physical space drops to around two per cent the system will initiate a complete shutdown. However, storage can be easily expanded on demand as new arrays can be connected or existing ones populated with more drives all on the fly.

Replication is a standard feature on all enterprise level storage solutions and Compellent doesn’t disappoint with the level of features on offer. You can create two separate and geographically remote SANs and replicate systems across them and both synchronous and asynchronous operations are supported. Plenty of choices are available as you can replicate entire volumes whilst replays can be transmitted to another SAN as well.

During testing we were very impressed with the Storage Center’s capabilities as it delivers a range of features that few vendors can match let alone beat at this level of the market. Compellent’s ace in the hole is its DBA technology which allows it to deliver the slick data progression capabilities and all supported with solid backup facilities and dynamic capacity expansion.


With support for both FC (Fibre Channel) and iSCSI targets the Storage Center compares extremely well on price with the established names in the storage industry. However, Compellent?s DBA technology and data progression features are quite unique making this a fine choice for network storage if you want automatic migration of data to lower cost storage.