RAID
Redundant Array of Independent Disks (RAID)
What is RAID?
RAID allows information to access several disks. RAID uses techniques such as disk striping (RAID Level 0), disk mirroring (RAID Level 1), and disk striping with parity (RAID Level 5) to achieve redundancy, lower latency, increased bandwidth, and maximized ability to recover from hard disk crashes.
RAID consistently distributes data across each drive in the array. RAID then breaks down the data into consistently-sized chunks (commonly 32K or 64k, although other values are acceptable). Each chunk is then written to a hard drive in the RAID array according to the RAID level employed. When the data is read, the process is reversed, giving the illusion that the multiple drives in the array are actually one large drive.
Who Should Use RAID?
System Administrators and others who manage large amounts of data would benefit from using RAID technology. Primary reasons to deploy RAID include:
- Enhances speed
- Increases storage capacity using a single virtual disk
- Minimizes disk failure
Hardware RAID versus Software RAID
There are two possible RAID approaches: Hardware RAID and Software RAID.
Hardware RAID
The hardware-based array manages the RAID subsystem independently from the host. It presents a single disk per RAID array to the host.
A Hardware RAID device connects to the SCSI controller and presents the RAID arrays as a single SCSI drive. An external RAID system moves all RAID handling “intelligence” into a controller located in the external disk subsystem. The whole subsystem is connected to the host via a normal SCSI controller and appears to the host as a single disk.
RAID controller cards function like a SCSI controller to the operating system, and handle all the actual drive communications. The user plugs the drives into the RAID controller (just like a normal SCSI controller) and then adds them to the RAID controllers configuration, and the operating system won’t know the difference.
Software RAID
Software RAID implements the various RAID levels in the kernel disk (block device) code. It offers the cheapest possible solution, as expensive disk controller cards or hot-swap chassis [1] are not required. Software RAID also works with cheaper IDE disks as well as SCSI disks. With today’s faster CPUs, Software RAID outperforms Hardware RAID.
The Linux kernel contains an MD driver that allows the RAID solution to be completely hardware independent. The performance of a software-based array depends on the server CPU performance and load.
To learn more about Software RAID, here are the key features:
- Threaded rebuild process
- Kernel-based configuration
- Portability of arrays between Linux machines without reconstruction
- Backgrounded array reconstruction using idle system resources
- Hot-swappable drive support
- Automatic CPU detection to take advantage of certain CPU optimizations
Configuring Software RAID
Users can configure Software RAID during the graphical installation process (Disk Druid), the text-based installation process, or during a kickstart installation.This chapter covers Software RAID configuration during the installation process using the Disk Druid application.
- Apply software RAID partitions to the physical hard drives.
To add a boot partition (/boot/) to a RAID partition, ensure it is on a RAID1 partiton.
- Creating RAID devices from the software RAID partitions.
- Optional: Configuring LVM from the RAID devices.
- Creating file systems from the RAID devices.
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