Designing the Right Data Storage Structure

A code vault that stores source code and enables programmers to check code in and out so that they can work on it locally on their systems probably doesn't justify a SAN with 28 platters. An Exchange 2003 Cluster that supports 3,000 users probably could. When designing a storage structure it is critical to understand your own needs.

Choosing the Right Connectivity

All the high-speed disks in the world won't amount to much if you can't get the data to and from the servers quickly. In a NAS environment, the network itself is the biggest concern for performance. Most NAS devices on the market use very fast heads that are literally dedicated computers with high performance processors and loads of memory. With SCSI RAID controllers on board, they can easily saturate multiple 100MB Ethernet connections. Attaching such a device to a low-end switch would result in the NAS running in an extremely restricted manner. Strongly consider using a switch that will enable you to use a gigabit connection.

Consider creating a separate network for the NAS environment. Say, for example, that the NAS is going to support a number of Exchange servers. By multi-homing the Exchange servers, they would have one Ethernet connection that faced the users and provided connectivity to the mail clients , whereas the other interface would be dedicated to NAS traffic. This would allow each interface to run unfettered by the traffic associated with the other network. This also enables you to upgrade only a subset of the network to improve performance and save money. The traffic of the database transaction back to the NAS device by Exchange would be much greater than the traffic associated with users viewing their mail because the traffic that would normally go to the local disk would now be traveling across the Ethernet via the Virtual Disk driver that connects the NAS to the Exchange server.

When selecting network gear for a NAS out-of- band network, focus on packets per second. Whenever possible, build this NAS network with multiple switches that are cross-connected. Connect each server to both switches with the NICs in a teamed mode. This will not only add bandwidth but also will create redundancy for the network layer. Odds are if the application warranted the use of a NAS device, it deserves redundancy at the network level as well.

Slicing and Dicing the Available Disk

Simple physics tells you that you'll get improvements in performance as you add more disks to an array. Because each drive's read/write head can operate simultaneously you get a fairly linear improvement as drives are added. NAS and SAN offer the advantage of dynamically increasing the size of a volume without taking the volume offline. This allows for the addition of even more spindles.

Although it's possible to later resize a volume from a NAS or SAN, you must be careful not to over subscribe the device. Devices that support snapshots of the data reserve twice the volume size that they claim for capacity. So in order to make 100GB available to a server, the NAS would reserve 200GB on itself. This ensures that it is able to complete all transactions. This function can be disabled on most devices but it is not recommended. This removes the protection from oversubscription of the disks.

Never Underestimate the Power of Multiple Spindles

Never underestimate the power of multiple spindles. For applications like Exchange that are very IO- intensive , a quad processor Xeon 1.6 GHz with 4GB of memory and seven local disks can support up to 1,000 heavy users. When the number of attached disks was increased to 112, the system was able to support more than 11,000 heavy users.