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Linux-Sysadmin.md

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related
cheat-sheet
LVM
RAID
Partitioning'
Security

Hardware

Info

Commands

lsusb
lspci
lsha
dmidecode
dmidecode -s system-product-name
hdparm -I /dev/....
smartctl -a /dev/...

Performance

I/O

  • fio: Synthetic IO stress test
  • iozone: Synthetic FS benchmark

I/O Scheduler

Get available and selected disk I/O scheduler

cat /sys/block/sda/queue/scheduler

Set Linux disk IO scheduler

echo deadline > /sys/block/sda/queue/scheduler

How to enable BFQ scheduler with udev

CPU

CPU Scheduler

Check and change scheduler settings with schedtool

schedtool <pid>                  # Print settings for given PID
schedtool -r                     # Show scheduler class prio ranges
schedtool -R -p 20 -e <command>  # Run command with SCHED_RR and nice 20

List per-process scheduler settings

ps -wweALo pid,spid,user,priority,ni,pcpu,vsize,time,args

Check CPU freq governor

cat /sys/devices/system/cpu/cpufreq/policy*/scaling_governor

Check SCHED_ISO settings

cat /proc/sys/kernel/iso_cpu

Check if CFS is default scheduler

grep cfs_rq /proc/sched_debug

Check if there are processes not running with CFS (SCHED_OTHER)

ps -ef|grep [0-9]|awk '\{system("chrt -p " $2);print $0}' | grep -Ev 'priority|SCHED_OTHER' |grep -A1 SCHED

Metrics

Human readable sar output

sar -n DEV 1 1 # e.g. show recent Device I/O 

# Save one complete sample if you have no collection enabled 
sar -o /tmp/sar.out -n ALL 1 1

Machine readable: you need to use sadf

sadf -j /tmp/sar.out -- -n DEV    # All in nice JSON

Metrics Theory

Swap Files

  • Create swap file and mount it

      dd if=/dev/zero of=/swapfile bs=1024000 count=1024          # Create ~1GB file
  mkswap /swapfile
  chmod 0600 /swapfile
  swapon /swapfile

    To persist mounting the swapfile add an entry in /etc/fstab like this

      /swapfile    none     swap    sw              0       0

Misc

  • Find broken links

      find . -xtype l

  • pv - Visualize pipe progress:

    pv -cN source < linux-3.4.0.tar.bz2 | bzcat | pv -cN bzcat | gzip -9 | pv -cN gzip > linux-3.4.0.tar.gz

  • Fix broken text encoding:

    iconv -c -t ASCII input.txt
iconv -c -t latin1 input.txt

  • rsyslog - Modify rate imux limiting

    $SystemLogRateLimitInterval 2
$SystemLogRateLimitBurst 50  # increase this

  • Running parallel SSH sessions using xargs

    echo "$hosts" | xargs --replace={} -t -n 1 -P 15 sh -c "ssh -o StrictHostKeyChecking=no -o PreferredAuthentications=publickey {} date || :"

  • Force line buffering (aka autoflush) for a process that does buffer logs

      exec stdbuf -oL -eL <command>