Troubleshooting (Ubuntu)

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Revision as of 15:28, 16 July 2012 by Sstrutt (talk | contribs) (→‎High System Load: Added other tools)
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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. Though if you expect peaks in load, then a high load over the last minute might not concern, whereas over 15mins it would.

top

The top command allows some basic insight into the system's performance, and is akin to the Task Manager in Windows. It probably won't provide the answer as to what the problem is, but it will probably allow you to focus in on the process(es) that are causing grief. 

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 Occassional 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="/home/user/load_log.txt"  # Update to a valid folder path
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 (update with correct path to load_log... 

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

Other Tools

If when using the above tools you're presented with disk/devices names of dm-0, dm-1, etc., which won't mean much. These are LVM logical devices, to understand what they map to use 

lvdisplay|awk  '/LV Name/{n=$3} /Block device/{d=$3; sub(".*:","dm-",d); print d,n;}'

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
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