10. Enterprise Linux Networking

In certain situations it is necessary for the networking infrastructure to have proper mechanisms to guarantee network availability nearly 100% of the time. Some related techniques are described in the following sections. Most of the following material can be found at the excellent Linas website: http://linas.org/linux/index.html and in the Linux High-Availability HOWTO

10.1 High Availability

Redundancy is used to prevent the overall IT system from having single points of failure. A server with only one network card or a single SCSI disk has two single points of failure. The objective is to mask unplanned outages from users in a manner that lets users continue to work quickly. High availability software is a set of scripts and tools that automatically monitor and detect failures, taking the appropriate steps to restore normal operation and to notifying system administrators.

10.2 RAID

RAID, short for Redundant Array of Inexpensive Disks, is a method whereby information is spread across several disks, using techniques such as disk striping (RAID Level 0) and disk mirroring (RAID level 1) to achieve redundancy, lower latency and/or higher bandwidth for reading and/or writing, and recoverability from hard-disk crashes. Over six different types of RAID configurations have been defined. There are three types of RAID solution options available to Linux users: software RAID, outboard DASD boxes, and RAID disk controllers.

• Software RAID: Pure software RAID implements the various RAID levels in the kernel disk (block device) code.
• Outboard DASD Solutions: DASD (Direct Access Storage Device) are separate boxes that come with their own power supply, provide a cabinet/chassis for holding the hard drives, and appear to Linux as just another SCSI device. In many ways, these offer the most robust RAID solution.
• RAID Disk Controllers: Disk Controllers are adapter cards that plug into the ISA/EISA/PCI bus. Just like regular disk controller cards, a cable attaches them to the disk drives. Unlike regular disk controllers, the RAID controllers will implement RAID on the card itself, performing all necessary operations to provide various RAID levels.

Related HOWTOs:

• http://metalab.unc.edu/mdw/HOWTO/mini/DPT-Hardware-RAID.html
• http://metalab.unc.edu/mdw/HOWTO/Root-RAID-HOWTO.html
• http://metalab.unc.edu/mdw/HOWTO/Software-RAID-HOWTO.html

RAID at linas.org:

• http://linas.org/linux/raid.html

10.3 Redundant networking

IP Address Takeover (IPAT). When a network adapter card fails, its IP address should be taken by a working network card in the same node or in another node. MAC Address Takeover: when an IP takeover occurs, it should be made sure that all the nodes in the network update their ARP caches (the mapping between IP and MAC addresses).

See the High-Availability HOWTO for more details: http://metalab.unc.edu/pub/Linux/ALPHA/linux-ha/High-Availability-HOWTO.html

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