Troubleshooting (Ubuntu)

High System Load

The system load is normally represented by the load average over the last 1, 5 and 15 minutes.

For example, the uptime command gives a single line summary of system uptime and recent load

user@server:~$ uptime
 14:28:49 up 9 days, 22:41,  1 user,  load average: 0.34, 0.36, 0.32

So in the above, as of 14:28:49 hrs the server has been up for 9 days 22 hours odd, has 1 user logged in, and the system load averages for the past 1, 5, and 15 minutes are shown.

The load average for a given period indicates how many processes were running or in a uninterruptable (waiting for IO) state. What's bad depends on your system, for a single CPU system a load average greater than 1 could be considered bad as there are more processes running than CPU's to service them.

top

The top command allows some basic insight into the system's performance, and is akin to the Task Manager in Windows.

user@server:~$ top
top - 14:32:09 up 9 days, 22:44,  1 user,  load average: 0.70, 0.44, 0.34
Tasks: 137 total,   1 running, 136 sleeping,   0 stopped,   0 zombie
Cpu(s): 93.8%us,  6.2%sy,  0.0%ni,  0.0%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:   1023360k total,   950520k used,    72840k free,    10836k buffers
Swap:  1757176k total,  1110228k used,   646948k free,   135524k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 6608 zimbra    20   0  556m  69m  12m S 69.1  6.9   0:03.26 java
17284 zimbra    20   0  649m 101m 3604 S  4.6 10.1  31:34.74 java
 2610 zimbra    20   0  976m 181m 3700 S  0.7 18.1 133:06.68 java
    1 root      20   0 23580 1088  732 S  0.0  0.1   0:04.70 init
    2 root      20   0     0    0    0 S  0.0  0.0   0:00.01 kthreadd
    3 root      RT   0     0    0    0 S  0.0  0.0   0:00.00 migration/0
....

Note that CPU metrics are with respect to 1 CPU, so on a multiple CPU system, seeing values > 100% is valid.

Overview of CPU Metrics, % over time

Code	Name	Description
us	User CPU	% of CPU time spent servicing user processes (excluding nice)
sy	System CPU	% of CPU time spent servicing kernel processes
ni	Nice CPU	% of CPU time spent servicing user nice processes (nice reduces the priority of process)
id	Idle CPU	% of CPU time spent idling (doing nothing)
wa	IO Wait	% of CPU time spent waiting for IO (high indicates disk/network bottleneck)
ha	Hardware Interrupts	% of CPU time spent servicing hardware interrupts
si	Software Interrupts	% of CPU time spent servicing hardware interrupts
st	Steal	% of CPU time stolen to service virtual machines

Task column heading descriptions (to change what columns are shown press f)

Key	Display	Name	Description
a	PID	Process ID	Task/process identifier
b	PPID	Parent PID	Task/process identifier of processes parent (ie the process that launched this process)
c	RUSER	Real User Name	Real username of task's owner
d	UID	User ID	User ID of task's owner
e	USER	User Name	Username ID of task's owner
f	GROUP	Group Name	Group name of task's owner
g	TTY	Controlling TTY	Device that started the process
h	PR	Priority	The task's priority
i	NI	Nice value	Adjusted task priority. From -20 meaning high priorty, through 0 meaning unadjusted, to 19 meaning low priority
j	P	Last Used CPU	ID of the CPU last used by the task
k	%CPU	CPU Usage	Task's usage of CPU
l	TIME	CPU Time	Total CPU time used by the task
m	TIME+	CPU Time, hundredths	Total CPU time used by the task in sub-second accuracy
n	%MEM	Memory usage (RES)	Task's usage of available physical memory
o	VIRT	Virtual Image (kb)	Task's allocation of virtual memory
p	SWAP	Swapped size (kb)	Task's swapped memory (resident in swap-file)
q	RES	Resident size (kb)	Task's unswapped memory (resident in physical memory)
r	CODE	Code size (kb)	Task's virtual memory used for executable code
s	DATA	Data+Stack size (kb)	Task's virtual memory not used for executable code
t	SHR	Shared Mem size (kb)	Task's shared memory
u	nFLT	Page Fault count	Major/Hard page faults that have occured for the task
v	nDRT	Dirty Pages count	Tasks memory pages that have been modified since last write to disk, and so can be readily freed from physical memory
w	S	Process Status	D - Uninterruptible sleep R - Running S - Sleeping T - Traced or Stopped Z - Zombie
x	Command	Command Line	Command used to start task
y	WCHAN	Sleeping in Function	Name (or address) of function that the task is sleeping in
z	Flags	Taks Flags	Task's scheduling flags

Identify Process Causing High System Load

If the high load is constant, just fire up top and see if there is a specific process to blame, or if your stuck waiting for disk or network IO.

If the high load is transient but repetitive, then you'll need to capture the output of top at the right time, the following script will create a log of top output during periods of high load

#!/bin/bash
#
# During high load, write output form top to file.
#
# Simon Strutt - July 2012

LOGFILE="load_log.txt"
MAXLOAD=100                     # Multiple by 100 as if comparison can only handle integers

LOAD=`cut -d ' ' -f 1 /proc/loadavg`
LOAD=`echo $LOAD '*100' | bc -l | awk -F '.' '{ print $1; exit; }'`     # Convert load to x100 integer

if [ $LOAD -gt $MAXLOAD ]; then
        echo `date '+%Y-%m-%d %H:%M:%S'`>> ${LOGFILE}
        top -b -n 1 >> ${LOGFILE}
fi

Schedule with something like...

crontab -e
1 * * * * /bin/bash  /home/simons/load_log

Network

No NIC

Especially after hardware changes, its possible the networking config no longer refers to the right interface.

1. Use ifconfig to confirm the current network config
2. Use dmesg | grep -i eth to ascertain what's been detected at boot time
3. Assuming it states that say eth0 has been changed to eth1 then just update the /etc/network/interfaces file

File System

Unable to Mount CD-ROM

Mounting drive with following command fails

• mount /dev/cdrom /media/cdrom/

If /media/cdrom/ doesn't exist

1. Create the file with mkdir /media/cdrom

If /dev/cdrom special device doesn't exist

1. Check for existing mappings and devices
   • ls -l /dev/ | grep cdrom
2. If an existing mapping exists but for a different drive number (eg cdrom2 -> sr0)
   • Then try mounting with that number
   • EG mount /dev/cdrom2 /media/cdrom/
3. If no existing mapping exists
   • Then try creating one for one of the listed devices
   • EG ln -sf /dev/sg0 /dev/cdrom

Replacing a Software RAID 1 Disk

This procedure was written from the following starting point...

• A machine originally with two disks in RAID1 has failed, one disk has been replaced, and machine started again

...and adapted from this post http://www.howtoforge.com/replacing_hard_disks_in_a_raid1_array

1. Backup whatever you can before proceeding, one mistake or system error could destroy your machine
2. Confirm which disk is new, and which is old (if the new disk is blank this is easy as there will be no partition info!)
   • fdisk -l
3. Partition the new disk the same as the original
   • sfdisk -d /dev/sda | sfdisk /dev/sdb
4. Confirm that the layout of both disks is now that same
   • fdisk -l
5. Add the newly created partitions to the RAID disks
   • mdadm --manage /dev/md0 --add /dev/sdb1
   • You may have more sd partitions than md partitions, the array size return through mdadm -D /dev/md* should roughly match the number of blocks found from fdisk -l
6. The arrays should now be being sync'ed, check progress by monitoring /proc/mdstat
   • more /proc/mdstat

SSH

Server Hostname Change

If the hostname (or IP) of the server you are SSH'ing to changes, the old entry needs to be removed from your SSH key known hosts file

• ssh-keygen -R <name or IP>

Packages

Errors etc received from apt-get

• Error 400 Bad Request
  • Somewhat misleadingly, the problem is normal caused by being unable to contact the update server. Consider adding proxy server config to your machine
• The following packages have been kept back
  • Package manager can hold back updates because they will cause conflicts, or sometimes because they're major kernel updates. Running aptitude safe-upgrade normally seems to force kernel updates through.

Reboot Required?

If a package update/installation requires a reboot to complete the following file will exist...

/var/run/reboot-required 

To see which packages caused this to be set, inspect the contents of...

/var/run/reboot-required.pkgs