The days when administrators could afford to simply throw more hardware at their network storage problems are long gone. To deliver the cost-effectiveness that enterprise’s crave, existing capacity must be managed much more efficiently, and Compellent’s Storage Center aims to do precisely that by providing a unique information lifecycle management (ILM) solution.
Compellent uses the concept of tiered storage where data is funnelled over Fibre Channel (FC) and IP storage area networks (SANs), then automatically migrated from high performance FC through to low cost Sata hard disks as it becomes less frequently used.
A key feature is Compellent’s data progression scheme, Dynamic Block Architecture (DBA), which creates up to three storage tiers and is capable of categorising physical hard disks based on their interfaces and spindle speeds. For Tier 1, a storage manager would employ 15,000rpm FC hard disks, but use lower cost 10,000rpm FC drives for Tier 2, while Tier 3 would comprise high capacity Sata drives. Data that is accessed frequently remains in Tier 1, but as it becomes less used it is moved to Tier 2 and then Tier 3.
For testing we were supplied with two 16-bay arrays, one populated with eight FC drives and the other with Sata disks. The arrays come with dual quad-port FC controllers and are linked to a head unit that comprises a Supermicro 3U rack system equipped with a 3.2GHz Xeon processor and 2GB of memory.
The Linux-based kernel runs on an IDE CompactFlash card and the chassis can be populated with multiple FC controllers or iSCSI host bus adapters (HBAs) with TCP offload engines (TOEs).
Compellent’s web management interface could be better designed to make it more accessible, but we did find initial configuration simple as all disk categorisation is done automatically and you can use multiple Raid levels for each tier.
An important feature of DBA is that it carries out data progression at the block level. All the storage manager or administrator needs to do is decide on the number of days a block has to remain unused before it is migrated, then pick a ratio that controls how data is moved down the tiers. This ratio also controls how quickly data is moved back up through the tiers if it becomes more frequently accessed.
Snapshots are the most basic form of backup in disk arrays but Compellent goes a stage further with its replay technology. DBA comes into play here as it is used to remember where blocks were when the snapshot, or replay, was taken and extra space is only used as new data is written to the volume. Replays are stored on Tier 3 by default, but can be placed in higher tiers for faster access if required.
Compellent’s dynamic capacity allows the creation of virtual volumes that are much larger than the physical capacity of the disks. Once again DBA makes its presence felt as it monitors the physical space in use and allocates blocks dynamically. Plenty of alerting facilities are on offer and when used space reaches 80 percent, the hardware controller will send out warnings via the management web interface plus multiple emails to administrators and even Compellent’s support staff as well. It is easy enough to increase capacity as extra arrays or more hard disks can be added on the fly.
