COP 5611 Operating Systems Spring 2010 Dan C. Marinescu Office: HEC 439 B Office hours: M-Wd 1:00-2:00 PM

Lecture 13 Reading Assignment: Chapter 8 from the online textbook Homework 3 due on March 3 Midterm: Wednesday March 17, the first week after Spring Break Last time: End-to-end-layer Resource Management - Congestion Today: Faults, Failures and Fault-Tolerant Design Measures of Reliability and Failure Tolerance Tolerating active Faults Next time 2 2 2 2 2

Reliable Systems from Unreliable Components Problem investigated first in mid 1940s by John von Neumann. Steps to build reliable systems Error detection Network protocols (link and end-to-end) Error containment – limit the effect of errors Enforced modularity: client-server architectures, virtual memory, etc. Error masking – ensure correct operation in the presence of errors Network protocols: error correction, repetition, interpolation for data cu real-time constrains

Faults and errors Fault a flaw with the potential to cause problems Software Hardware Design Implementation Operation Environment Types of faults Latent Active Error  the consequence of an active fault.

Error containment in a layered system Several design strategies are possible. The layer where an error occurs: Masks the error  correct it internally so that the higher layer is not aware of it. Detects the error and report its to the higher layer  fail-fast. Stops  fail-stop. Does nothing. Types of faults Transient (caused by passing external condition)/Persistent Soft /Hard  Can be masked or not by a retry. Intermittent  occurs only occasionally and it is not reproducible Latency of a fault – time until a fault causes an error A long latency may allow errors to accumulate and defeat periodic error correction

The fault-tolerance design process is iterative Begin the design of a fault-tolerant model Identify potential faults Estimate the risk of each one Design methods to detect the errors for the highest risk faults. Design methods to deal with the errors for the highest risk faults Contain the damage from high risk errors through modularity. Design procedures to contain the errors detected by: Temporal redundancy (retry the operation) Spatial redundancy (deploy multiple components) Update the model to account for the error masking procedures Iterate until the probability of un-tolerated faults is small Observe the system in the real world Study the error logs Identify the cause of each error Use the information collected to improve the model and iterate again

Measures of reliability TTF – time to failure MTTF – mean time to failure MTTF = 1/N ∑ TTFi TTR – time to repair MTTR – mean time to repair MTTR = 1/N ∑ TTRi MTBF – mean time between failures MTBF =MTTF + MTTR Availability =MTTF/MTBF Down time = ( 1- Availability) = MTTR/MTBF

The conditional failure rate

Reliability functions Unconditional failure rate f(t) = Pr(module fails between t and t = dt) Reliability R(t) = Pr(module functions at time t given that it was functioning at time 0). This function is memoryless

Active faults How to respond Errors Do nothing Fail-fast – report the error Fail-safe – operate in an acceptable manner (e.g., stop sign/flashing lights) Fail-soft – operate correctly with reduced performance Mask the error Errors Detectable  the error can be reliably detected. Maskable  Correctable Untolerated error  undetactable, dndetected, unmaskable, unmasked.

Fault tolerance models Categorize all errors Evaluate the probability of occurrence of each error. Modify the design to make the most likely errors detectable. Implement a detection procedure for each detectable error and identify the modules in which it is detected as fail-fast. Try to devise masking procedure for each detectable error. Evaluate the probability of occurrence, the cost of the masking procedure and the cost of failure for each detectable error

Errors in analog and digital systems Analog systems  the designer specifies a tolerance to deal with small errors. Digital system Spatial redundancy Temporal redundancy Coding Forward error-correction.