Managing RAID on Linux - Derek Vadala [11]
Figure 2-2. Disk controllers shift the array functions off the CPU, yielding an increase in performance.
While a lot of card manufacturers have recently begun to support Linux, it's important to make sure that the card you're planning to purchase is supported under Linux. Be sure that your manufacturer provides at least a loadable kernel module, or, ideally, open source drivers that can be statically compiled into the kernel. Open source drivers are always preferred over binary-only kernel modules. If you are stuck using a binary-only module, you won't get much support from the Linux community because without access to source code, it's quite impossible for them to diagnose interoperability problems between proprietary drivers and the Linux kernel. Luckily, several vendors either provide open source drivers or have allowed kernel hackers to develop their own. One shining example is Mylex, which sells RAID controllers. Their open source drivers are written by Leonard Zubkoff [2] of Dandelion Digital and can be managed through a convenient interface under the /proc filesystem. Chapter 5 discusses some of the cards that are currently supported by Linux.
Outboard solutions
The second hardware alternative is a turnkey solution, usually found in outboard drive enclosures. These enclosures are typically connected to the system through a standard or high-performance SCSI controller. It's not uncommon for these specialized systems to support multiple SCSI connections to a single system, and many of them even provide directly accessible network storage, using NFS and other protocols.
These outboard solutions generally appear to an operating system as a standard SCSI block device or network mount point (see Figure 2-3) and therefore don't usually require any special kernel modules or device drivers to function. These solutions are often extremely expensive and operate as black box devices, in that they are almost always proprietary solutions. Outboard RAID boxes are nonetheless highly popular among organizations that can afford them. They are highly configurable and their modular construction provides quick and seamless, although costly, replacement options. Companies like EMC and Network Appliance specialize in this arena.
Figure 2-3. Outboard RAID systems are internally managed and connected to a system to which they appear as a single hard disk.
If you can afford an outboard RAID system and you think it's the best solution for your project, you will find them reliable performers. Do not forget to factor support costs into your budget. Outboard systems not only have a high entry cost, but they are also costly to maintain. You might also consider factoring spare parts into your budget, since a system failure could otherwise result in downtime while you are waiting for new parts to arrive. In most cases, you will not be able to find replacement parts for an outboard system at local computer stores, and even if they are available, using them will more than likely void your warranty and support contracts.
I hope you will find the architectural discussions later in this chapter helpful when choosing a vendor. I've compiled a list of organizations that provide hardware RAID systems in the Appendix. But I urge you to consider the software solutions discussed throughout this book. Administrators often spend enormous amounts of money on solutions that are well in excess of their needs. After reading this book, you may find that you can accomplish what you set out to do with a lot less money and a little more hard work.
Storage Area Network (SAN)
SAN is a relatively new method of storage management, in which various storage platforms are interconnected on a separate, usually high-speed, network (see Figure 2-4). The SAN is then connected to local area networks (LANs) throughout an organization. It is not uncommon for a SAN to be connected to several different parts of a LAN so that users do not share a single path to the SAN. This prevents a network bottleneck and allows better throughput between users and