Compute capacity and survival for twelve array configurations, then answer four outage questions

applied · 80 min · Objective 3.1

Task

Do the arithmetic the exam asks for until it is automatic, then apply it to outage scenarios -- because the exam's harder questions are not 'what is the capacity' but 'would this configuration have survived this failure'.

Steps

  1. Write lab/raid/configs.csv with header id,level,disks,disk_tb,usable_tb,survives and twelve rows covering levels 0, 1, 5, 6 and 10 with varying disk counts and sizes.
  2. Fill usable_tb and survives by hand first, then verify at least six rows with a calculation written into lab/raid/check.md showing the formula used.
  3. Write lab/raid/outages.txt with four failure scenarios, one per line -- for example 'two disks fail within an hour', 'one disk fails and a second develops an unreadable sector during the rebuild'.
  4. For each scenario and each of four chosen configurations, record in lab/raid/survival.csv with header scenario,config_id,survived,why whether the data survives.
  5. Write lab/raid/not-backup.md listing four events an array does not protect against, with the mechanism for each, and naming what does.

Verify

awk -F, 'NR>1 && NF>=6 {n++} END {print n" configuration(s)"}' lab/raid/configs.csv
awk -F, 'NR>1 {print $2}' lab/raid/configs.csv | sort -u | wc -l
grep -c . lab/raid/check.md
grep -c . lab/raid/outages.txt
awk -F, 'NR>1 && NF>=4 {n++} END {print n" survival verdict(s)"}' lab/raid/survival.csv
awk -F, 'NR>1 && tolower($3)=="no" {n++} END {print n+0" configuration(s) that did NOT survive"}' lab/raid/survival.csv
grep -c . lab/raid/not-backup.md

Twelve configurations across four or more levels, six verified by formula, four scenarios, sixteen survival verdicts of which at least four are failures, and four events the array does not cover. A survival table where everything survives has not been given a hard enough scenario.

Notes

The rebuild scenario is the one worth dwelling on. A RAID 5 array with one failed member and an unreadable sector on a survivor is the configuration that loses data most often in practice, and the arithmetic does not show it.

This is an independent study companion for CompTIA A+ Core 1 220-1201 and is not produced by or endorsed by CompTIA.