Tiered SAN means better storage
With Compellent, you can mix and match drives and more
- By Greg Crowe
- Mar 15, 2006
The Compellent Storage Center uses a Dynamic Block Architecture for managing data.
The right storage solution has always been the brass ring of running a network. As users' needs grow, and the files they work with become bulkier and more complex, your storage requirements increase.
And capacity isn't the only concern. A user wants a file this instant, no matter when it was last modified. This has to be balanced with staying within budget, and using the least expensive option when possible.
A backup-to-tape solution is initially inexpensive and cheaply expandable, but it can be cumbersome to maintain. A storage area network is arguably a better, faster solution.
The Storage Center from Compellent is a good example of the advantages a SAN can bring to your network. Its methods of data management and expandability bring network storage to a whole new level.
Compellent works through resellers whose technicians come in to take you through the entire set-up process. Depending on the complexity of both the system and the client's data, this can take a couple of hours to two days from start to finish. Setup is included in the cost of the system.What we looked at
A minimal setup for the Storage Center (which is what we tested here in the GCN Lab) comes with one server connected to two 16-slot drive bays, one for Fibre Channel drives and the other for cheaper serial ATA drives. This gave us a total disk space of three terabytes using just half of the available slots.
The Web-based interface can be accessed from any computer on the network. You can see which physical drives are in which bays, and start to manage disk folders or groups of disks (we put all 16 disks in one folder). The system shows you the disk tiers it's created based on disk performance, with Tier One being the fastest and Tier Three the slowest. Each tier is broken down into three RAID levels: 10, 5 (five-disk), and 5 (nine-disk).
At this point you decide which types of storage you will make available to the logical volumes you'll be setting up. When you create a volume, which LAN users will access as a logical drive, you select tiers and RAID level for the drive.
When a file is created on the volume, the Storage Center uses the fastest tier/RAID selection to store it. After a certain number of days (the default is 12, but this can be changed in the control center), it automatically migrates to the second-fastest location. So it may at first store the file in the faster, RAID 10 tier. Then, after 12 days the systems could store it as RAID 5 (five-disk) in the same or slower tier, depending on your specifications. When the file is again accessed, it migrates back to Tier One. It does this automatically, changing RAID allocations on the fly as needed.
On-the-fly tiered storage is accomplished through a technology called Dynamic Block Architecture. Each block of each drive is manipulated separately, and metadata is kept for each block. This metadata includes modification times, access frequency, disk type, tier, RAID level and associated volume. Using this information, data can be switched to another tier or RAID type quickly, without affecting performance. It takes longer than a couple of seconds to make these changes only when the majority of the data on a disk is changing tiers.
Another feature we found useful was Data Instant Replay. The server takes snapshots of the data allocation at intervals you set in the control panel. Only the blocks containing actual data are snapped, saving a great deal of disk space per snapshot. If something happens to your SAN, such as a virus, you can restore the system from a previous snapshot. While having snapshot capability is fairly standard for a storage solution, the Storage Center makes it easy to use and allows you to keep many snapshots at once.
Anyone who installs hard drives will tell you that having one fail is just a matter of installing enough drives. In the case of a managed system, what matters is how the system deals with failure. Our Storage Center noticed that one of the drives was failing and immediately contacted Compellent, who called to inform us and schedule a technician to replace the drive.
Compellent calls this Copilot Services. The Storage Center is set up to 'phone home' to Compellent via a local e-mail server at regular intervals, enabling technicians to spot a problem and contact the client if necessary. There is an annual fee based on the size of your system, but it's well worth it.
The Storage Center is expandable, limited only by the client's desires and budget. You can start out with a minimal configuration like the one we had and add bays, servers and switches.
Because data is manipulated at a block level, replication between the primary system and remote sites can be asynchronous, with only the altered blocks being synchronized. This can save quite a bit of money, especially if you have multiple remote sites.
That said, the base price of the system we tested is rather prohibitive, but as the system gets larger, the price per terabyte drops. On an extremely large scale, this can turn into quite an economical decision.
Is the Compellent Storage Center right for you? For fewer than two or three terabytes, probably not. But for larger-scale storage needs, a managed SAN is almost mandatory, and the Storage Center might just fit the bill.