Scenario

Users are complaining about slow file access. System metrics show high disk utilization.

Task

Reduce I/O activity of top offender using I/O priorities to idle. Keep critical jobs (databases, message queues, applications) at high priority.

Example

 # Before (high I/O contention)
 
 Total DISK READ:      45.67 M/s | Total DISK WRITE:     123.45 M/s

   TID  PRIO  USER      DISK READ  DISK WRITE  COMMAND
  5678  be/4  backup     2.34 M/s   98.76 M/s  rsync /data /backup
  3421  be/4  postgres  15.23 M/s   12.45 M/s  postgres: vacuum
  8234  be/4  appuser    8.45 M/s    3.21 M/s  /usr/bin/log-processor
  
 # After (non-critical jobs throttled)
 
 Total DISK READ:      18.34 M/s | Total DISK WRITE:      35.21 M/s

   TID  PRIO  USER      DISK READ  DISK WRITE  COMMAND
  5678  idle  backup     0.50 M/s    2.10 M/s  rsync /data /backup
  3421  be/4  postgres  15.23 M/s   12.45 M/s  postgres: vacuum
  8234  idle  appuser    1.20 M/s    0.80 M/s  /usr/bin/log-processor

Step 1: Identify processes with high I/O activity

sudo iotop -o -b -n 10

Observe the processes performing I/O. From the output, note disk write activity:

• job1_backup.dat → writes 50-99 MB/s
• job2_database.dat → writes 30-60 MB/s (critical - must not throttle)
• job3_logs.datc→ writes 20-40 MB/s

Step 2: Note the TIDs for non-critical jobs
From the iotop output above, identify and note down:

• job1_backup.dat TID: (e.g., 1234)
• job3_logs.dat TID: (e.g., 9012)

These are the processes you will throttle.

Step 3: Check current I/O priority

sudo ionice -p <TID>

Replace with the actual TID from Step 1. Output shows:
none: prio 0

This means default best-effort class with priority 0.

Step 4: Set I/O priority to idle class

sudo ionice -c 3 -p <TID_JOB1>
sudo ionice -c 3 -p <TID_JOB3>

Replace TIDs with actual values.

Step 5: Verify I/O priority changed and activity reduced

sudo iotop -o -b -n 3

After applying idle class to both job1 and job3, observe:

TID  PRIO  USER     DISK READ  DISK WRITE
2166 idle  root     2.49 M/s   0.00 B/s    ← job1: Priority "idle", I/O reduced
2188 idle  root     1.50 M/s   0.00 B/s    ← job3: Priority "idle", I/O reduced
2185 be/4  root     0.00 B/s   89.57 M/s   ← job2 (critical): Still be/4, full speed

Notice:

• Both job1 and job3 show priority idle (not be/4)
• Their I/O dropped dramatically (from 50-99 MB/s and 20-40 MB/s to ~1-3 MB/s)
• Critical job2 remains at be/4 with full I/O bandwidth