Managing RAID on Linux - Derek Vadala [10]
Linux, on the other hand, has a really good chance to change the negative perceptions of software RAID. Not only is Linux's software RAID open source, the inexpensive hardware that runs Linux finally makes it easy and affordable to build reliable software RAID systems. Administrators can now build systems that have sufficient processing power to deal with day-to-day user tasks and high-performance system functions, like RAID, at the same time. Direct access to developers and a helpful user base doesn't hurt, either.
If you're still not convinced that software RAID is worth your time, then don't fret. There are also plenty of hardware solutions available for Linux.
Hardware
Hardware RAID means that arrays are managed by specialized disk controllers that contain RAID firmware (embedded software). Hardware solutions can appear in several forms. RAID controller cards that are directly attached to drives work like any normal PCI disk controller, with the exception that they are able to internally administer arrays. Also available are external storage cabinets that are connected to high-end SCSI controllers or network connections to form a Storage Area Network (SAN). There is one common factor in all these solutions: the operating system accesses only a single block device because the array itself is hidden and managed by the controller.
Large-scale and expensive hardware RAID solutions are typically faster than software solutions and don't require additional CPU overhead to manage arrays. But Linux's software RAID can generally outperform low-end hardware controllers. That's partly because, when working with Linux's software RAID, the CPU is much faster than a RAID controller's onboard processor, and also because Linux's RAID code has had the benefit of optimization through peer review.
The major trade-off you have to make for improved performance is lack of support, although costs will also increase. While hardware RAID cards for Linux have become more ubiquitous and affordable, you may not have some things you traditionally get with Linux. Direct access to developers is one example. Mailing lists for the Linux kernel and for the RAID subsystem are easily accessible and carefully read by the developers who spend their days working on the code. With some exceptions, you probably won't get that level of support from any disk controller vendor—at least not without paying extra.
Another trade-off in choosing a hardware-based RAID solution is that it probably won't be open source. While many vendors have released cards that are supported under Linux, a lot of them require you to use closed-source components. This means that you won't be able to fix bugs yourself, add new features, or customize the code to meet your needs. Some manufacturers provide open source drivers while providing only closed-source, binary-only management tools, and vice versa. No vendors provide open source firmware. So if there is a problem with the software embedded on the controller, you are forced to wait for a fix from the vendor—and that could impact a data recovery effort! With software RAID, you could write your own patch or pay someone to write one for you straightaway.
RAID controllers
Some disk controllers internally support RAID and can manage disks without the help of the CPU (see Figure 2-2). These RAID cards handle all array functions and present the array as a standard block device to Linux. Hardware RAID cards usually contain an onboard BIOS that provides the management tools for configuring and maintaining arrays. Software packages that run at the OS level are usually provided as a means of post-installation array management. This allows administrators to maintain RAID devices without