Designing High Availability Networks, Systems, and Software

for the University Environment

Deke Kassabian and Shumon HuqueThe University of Pennsylvania

January 14, 2004

About Penn

The University of Pennsylvania was founded by Ben Franklin in 1751

Penn is part of the Ivy League Located in western Philadelphia Community of more than 30,000 people

General Goals Networked services available as expected

by our users Minimized time to repair (TTR) for when

outages do occur Ability to perform maintenance and

upgrades (planned downtime) non-disruptively

Cost effectiveness in meeting these goals

Definitions

Availability High Availability (HA) Rapid Recovery (RR) Disaster Recovery (DR) Basic Systems

Definitions

Disaster Recovery (DR) -The process of restoring a service to full operation after an interruption in service

Definitions

Basic System - a Basic System is a {Network, System, Service} with only the most basic of protections against outages

Examples: A network recoverable using spare parts A single computer system with RAID disk A service recoverable from tape backups

Definitions

Availability - the percentage of total time that a {Network, System, Service} is available for use

Related points: Advertised periods of availability Availability as advertised Absolute availability

Definitions

High Availability (HA) - a {Network, System, Service} with specific design elements intended to keep availability above a high threshold (eg, 99.99%)

Definitions

Rapid Recovery (RR) - a {Network, System, Service} with specific design elements intended to recover from downtime very quickly (eg, 15 minutes)

Metrics Economics of high availability (the

costs of non-available) Calculating availability How availability measurements are

performed

Economics of high availability What is the cost of an outage in your

Student Courseware systems and student record systems

Financial systems Primary campus web site and Email servers DNS, DHCP and AuthN systems Internet connection(s) Development / Gifts systems

How much should you be willing to spend to minimize downtime of any or all of these?

Calculating availability

Availability can be measured directly through periodic polling (eg, SNMP, Mon, Nagios)

A formula for predicting availability of a single component

MTBF(MTBF+TTR) 1 TTR

(MTBF+TTR)or

Design Principals Towards HA

Minimize points of catastrophic failure Maximize redundancy Minimize fault zones Minimize complexity and cost

Applying the above principles to Networks Systems Services

Specific examples at Penn High Availability Services Rapid Recovery Services

High Availability Design Strategies employed to achieve HA:

Server redundancy Hardware component redundancy Storage redundancy (RAID) Network redundancy Redundant power, A/C, cooling etc Application protocols that can transparently

failover to alternate servers Secondary offsite hosting (of some services like

DNS)

Rapid Recovery Design Strategies employed to achieve RR:

Standby servers and storage Some HA design elements:

Hardware redundancy, storage redundancy, network redundancy, power, A/C redundancy etc

Note: services deployed in the RR model typically don’t have an easy way to transparently failover to alternate servers (eg. E-mail, Web etc)

Network Aggregation Point Abbreviation: NAP Machine rooms in separate campus locations

that house critical network electronics and servers.

Good environmentals and extensive connectivity to campus fiber-optic cable plant

Both HA and RR services utilize multiple NAPs

Central Infra. Networks AKA “NOC Networks” (historical name) 3 highly redundant IP networks that house systems

providing critical infrastructure services Each network is triply connected to campus routing

core via distinct NAP locations Network wiring traverses physically diverse fiber

conduit pathways Use of router redundancy protocols (VRRP) & Layer-

2 path redundancy (802.1D) for high availability

HA Server Platforms Two sets of three replicated servers

3 KDC servers: central authentication 3 NOC servers: everything else

Kerberos runs on separate systems mainly for security reasons.

High Availability: KDCs KDCs (3):

3 distinct machines (kdc1, kdc2, kdc3) Run only Kerberos AS and TGS Each located in a different campus machine room Each connected to a distinct IP network

Via a distinct IP core router Additionally each network is triply connected to the

campus routing core via 3 NAPs

High Availability: NOCs 3 “NOC” systems (a historical name)

Provide: DNS, DHCP, NTP, RADIUS plus a few homegrown services

Same physical and network connectivity as the KDCs

In addition: some servers have a secondary interface on a different NOC network (for reasons to be explained later)

HA Application Failover Kerberos DNS RADIUS NTP DHCP

Current spec supports only 2 failover systems

Non-HA homegrown services: PennNames

Rapid Recovery service Example: E-mail and Web service A set of servers and storage is replicated at two sites: primary

and standby Primary site: active servers and storage Secondary site: standby servers and replicated storage Data from 1st site is synchronously replicated to 2nd Two separate fibrechannel networks interconnect systems and

storage at both sites Catastrophic failure event: system can be manually reconfigured

to use the standby servers and/or secondary storage ( ~ 30 minutes)

Servers are located on the HA primary infrastructure network

Experiences at Penn Where these approaches have been helpful

Higher availability, non-disruptive maintenance Where they have not

Complexity can be hard to manage! Where cost has been high

Replicated systems and networks, high-end storage solutions

Real availability experience DNS, a critical service, went from 99.0% to

99.999% availability!

Future Enhancements Making RR services highly available:

“clustering”, IETF rserpool etc Metropolitan area DR (or better) Rolling disaster protection Others:

IP Multipathing Trunking links to servers

802.3ad, SMLT, DMLT or similar Rapid Spanning Tree (IEEE 802.1w) Multi-master KADM service

Improved management and monitoring infrastructure

Feedback

Questions, comments Your designs, experiences, successes

Contact Info:deke@isc.upenn.edushuque@isc.upenn.edu