Software Raid; Raid Levels And Linear Support - Red Hat ENTERPRISE LINUX 5 - DEPLOYMENT Deployment Manual

Chapter 4. Redundant Array of Independent Disks (RAID)

4.3.2. 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
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

4.4. RAID Levels and Linear Support

RAID supports various configurations, including levels 0, 1, 4, 5, and linear. These RAID types are
defined as follows:
• Level 0 — RAID level 0, often called "striping," is a performance-oriented striped data mapping
technique. This means the data being written to the array is broken down into strips and written
across the member disks of the array, allowing high I/O performance at low inherent cost but
provides no redundancy. The storage capacity of a level 0 array is equal to the total capacity of the
member disks in a Hardware RAID or the total capacity of member partitions in a Software RAID.
• Level 1 — RAID level 1, or "mirroring," has been used longer than any other form of RAID. Level 1
provides redundancy by writing identical data to each member disk of the array, leaving a "mirrored"
copy on each disk. Mirroring remains popular due to its simplicity and high level of data availability.
Level 1 operates with two or more disks that may use parallel access for high data-transfer rates
when reading but more commonly operate independently to provide high I/O transaction rates. Level
1 provides very good data reliability and improves performance for read-intensive applications but at
a relatively high cost. The storage capacity of the level 1 array is equal to the capacity of one of the
mirrored hard disks in a Hardware RAID or one of the mirrored partitions in a Software RAID.
Note
RAID level 1 comes at a high cost because you write the same information to all of the
disks in the array, which wastes drive space. For example, if you have RAID level 1 set
up so that your root (/) partition exists on two 40G drives, you have 80G total but are
only able to access 40G of that 80G. The other 40G acts like a mirror of the first 40G.
A hot-swap chassis allows you to remove a hard drive without having to power-down your system.
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are not required.