Managing RAID on Linux - Derek Vadala [83]
Like any standard motherboard BIOS, most controllers follow a fairly standard menu system, with the following conventions:
Arrow keys and/or TAB move between fields.
ENTER confirms, enters a submenu, or selects the current field.
ESC moves to the previous menu or cancels the current dialog.
PLUS, MINUS, and SPACEBAR change values in the current field.
Some fields might require normal alphanumeric input, and some controllers may adhere to only a subset of these conventions.
System Installation
Normally you need to configure a hardware array using the controller's BIOS before you can install an operating system. However, there are a few exceptions when creating an array first is undesirable.
The first case is when you already have an operating system on a disk that is connected to the controller. This usually happens when you upgrade an existing system by installing a RAID controller. Some controllers support a pass-through mode, which means that you can connect an existing system disk to a new controller and boot the system normally, without any additional configuration. However, not all controllers support this pass-through mode. In that case, you first need to configure the existing system disk as a JBOD mode array. I've included cookbook examples for setting up JBOD modes for existing system disks in many of the sections about individual controllers, later in this chapter.
Since many users upgrade from a single disk environment to a RAID-1 configuration, I have also included cookbook examples that will help you migrate from a single disk system to RAID-1, using the controllers that I have evaluated. These procedures are often not documented, even though they represent a common task that many users and system administrators need to perform.
Unfortunately, it's really not possible to move from a single disk system to any RAID level other than JBOD or disk mirroring. Although moving from a single disk to a RAID-0 or RAID-5 might be highly desirable, it is nearly impossible without a backup and recovery step. Unfortunately, some manufacturers imply that these procedures are possible in the way they name their menu items and document the controllers. I'll point out any potential workarounds or migration hazards that I'm aware of in the sections on individual controllers.
If you plan to use arrays only for data drives, you can safely install the operating system onto a single disk and create arrays later. You might need first to create a JBOD mode array that refers to the disk onto which you plan to install the system. When you are ready to create arrays, either reboot the system and use the controller's BIOS or use user-space utilities, if available.
RAID Autoconfiguration
Many RAID controllers come with automatic configuration features. These features are designed to make it easy for users who have no background in RAID to help determine which RAID level best meets their needs. I hope that, having purchased this book, you never need to use these features. Instead, configure your arrays manually, using the material from earlier chapters as a reference.
Autoconfiguration works by asking the user a series of questions that are used to determine performance and redundancy requirements. For example, the controller might prompt the user with the question:
Do you need fault tolerance?
Depending on whether the user answers yes or no, the controller can determine which RAID levels to eliminate from the possible choices. An answer of "no" eliminates RAID-1 and RAID-5, while answering "yes" includes them, but excludes RAID-0 and JBOD. A follow-up question might look like this:
Do you need a spare disk?
Depending on how this question is answered, the controller could determine how to size a potential