Tag: systemd

The core dump file in Arch Linux

Arch Linux uses systemd, and the core dump files will be stored in /var/lib/systemd/coredump directory by default.

Let’s look at the following simple program:

int main(void) {
        int *p = 0;
        *p = 1;
}

Compile and run it:

$ gcc -g -o test test.c
$ ./test
Segmentation fault (core dumped)

Definitely, the program crashes! Check the core dump files:

$ coredumpctl list
TIME                            PID   UID   GID SIG COREFILE EXE
......
Mon 2017-01-09 16:13:44 SGT    7307  1014  1014  11 present  /home/xiaonan/test
$ ls -alt /var/lib/systemd/coredump/core.test*
-rw-r-----+ 1 root root     22821 Jan  9 16:13 /var/lib/systemd/coredump/core.test.1014.183b4c57ccad464abd2eba2c104a47a8.7307.1483949624000000000000.lz4

According to the file name and time, we can find the program’s corresponding core dump binary. To debug it, we can use “coredumpctl gdb PID” command:

$ coredumpctl gdb 7307
       PID: 7307 (test)
       UID: 1014 (xiaonan)
       GID: 1014 (xiaonan)
    Signal: 11 (SEGV)
 Timestamp: Mon 2017-01-09 16:13:44 SGT (6min ago)
Command Line: ./test
Executable: /home/xiaonan/test
 Control Group: /user.slice/user-1014.slice/session-c4.scope
          Unit: session-c4.scope
         Slice: user-1014.slice
       Session: c4
     Owner UID: 1014 (xiaonan)
       Boot ID: 183b4c57ccad464abd2eba2c104a47a8
    Machine ID: 25671e5feadb4ae4bbe2c9ee6de97d66
      Hostname: supermicro-sys-1028gq-trt
       Storage: /var/lib/systemd/coredump/core.test.1014.183b4c57ccad464abd2eba2c104a47a8.7307.1483949624000000000000.lz4
       Message: Process 7307 (test) of user 1014 dumped core.

            Stack trace of thread 7307:
            #0  0x00000000004004b6 main (test)
            #1  0x00007f67ba722291 __libc_start_main (libc.so.6)
            #2  0x00000000004003da _start (test)

GNU gdb (GDB) 7.12
Copyright (C) 2016 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-pc-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
<http://www.gnu.org/software/gdb/documentation/>.
For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from /home/xiaonan/test...done.
[New LWP 7307]
Core was generated by `./test'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x00000000004004b6 in main () at test.c:3
3               *p = 1;
(gdb) bt
#0  0x00000000004004b6 in main () at test.c:3

BTW, if you omit PID, “coredumpctl gdb” will launch the newest core dump file.

Attention should be paid. The core dump size is limited to 2GiB, so if it is too large, it will be truncated, and generate the warning like this (Please refer this bug):

......
BFD: Warning: /var/tmp/coredump-ZrhAM4 is truncated: expected core file size >= 2591764480, found: 2147483648.
......

References:
GDB and trouble with core dumps;
Core dump.

Deploy Docker Swarm cluster on one host

Sometimes, you just want to learn the internal mechanics of Docker Swarm, but unfortunately there is only one Linux box at hand, and you don’t want to bother to install Virtual Machines on it. In this scenario, you certainly can build a Docker Swarm cluster on one host, and this tutorial will provide a detailed guide:

(1) Make sure the Go environment has been ready on your system, if not, please follow this document to setup it. Also remember add$GOPATH/bin into $PATH environment variable.

(2) Install Docker Swarm:

# go get -u github.com/docker/swarm

Execute swarm command to check whether Docker Swarm is well equipped:

# swarm
Usage: swarm [OPTIONS] COMMAND [arg...]

A Docker-native clustering system

Version: 1.2.3 (HEAD)

Options:
  --debug                       debug mode [$DEBUG]
  --log-level, -l "info"        Log level (options: debug, info, warn, error, fatal, panic)
  --experimental                enable experimental features
  --help, -h                    show help
  --version, -v                 print the version
......

(3) Modify the Docker configuration file. E.g., on my RHEL 7, the file is /etc/sysconfig/docker:

# systemctl show docker
......
EnvironmentFile=/etc/sysconfig/docker (ignore_errors=yes)
......

Add “-H tcp://127.0.0.1:2375” in OPTIONS field:

# cat /etc/sysconfig/docker
# /etc/sysconfig/docker

# Modify these options if you want to change the way the docker daemon runs
OPTIONS='--selinux-enabled -H tcp://127.0.0.1:2375 -H unix:///var/run/docker.sock'

Restart Docker, and check whether the new OPTIONS takes effect:

# systemctl restart docker
# systemctl status docker
● docker.service - Docker Application Container Engine
   Loaded: loaded (/usr/lib/systemd/system/docker.service; disabled; vendor preset: disabled)
   Active: active (running) since Wed 2016-06-08 12:32:19 CST; 10s ago
     Docs: http://docs.docker.com
 Main PID: 14429 (sh)
   CGroup: /system.slice/docker.service
           ├─14429 /bin/sh -c /usr/bin/docker-current daemon $OPTIONS            $DOCKER_STORAGE_OPTIONS            $DOCKER_NETWORK_OPTI...
           ├─14430 /usr/bin/docker-current daemon --selinux-enabled -H tcp://127.0.0.1:2375 -H unix:///var/run/docker.sock --add-registr...
           └─14431 /usr/bin/forward-journald -tag docker
......

(4) Run “swarm create” command to create token for the cluster:

# swarm create
d10eacbda9763b0740548a2a4c2f1a59

(5) Execute swarm join to create a Docker Swarm node:

# swarm join --addr 127.0.0.1:2375 token://d10eacbda9763b0740548a2a4c2f1a59
INFO[0000] Registering on the discovery service every 1m0s...  addr=127.0.0.1:2375 discovery=token://d10eacbda9763b0740548a2a4c2f1a59
......

You should notice that the argument of --addr option is the IP and port of the Docker engine on this host. Since we have set theOPTIONS in Docker configuration file in step 3, the IP should be 127.0.0.1 whilst port is 2375.

(6) Open a new terminal, and create the manager of the cluster. Because port 2375 is occupied by Docker engine, we use another available port:

# swarm manage -H 127.0.0.1:3375 token://d10eacbda9763b0740548a2a4c2f1a59
INFO[0000] Listening for HTTP                            addr=127.0.0.1:3375 proto=tcp
INFO[0001] Registered Engine localhost.localdomain at 127.0.0.1:2375

Through the log, you can see the node and manager have communicated successfully.

Now, you can think a Docker engine is listening on tcp://127.0.0.1:3375, but actually, there is one Docker cluster behindtcp://127.0.0.1:3375, even though the cluster has only one node. You can play docker client commands now, such as get the cluster info:

# docker -H tcp://127.0.0.1:3375 info
Containers: 0
Images: 5
Server Version: swarm/1.2.3
Role: primary
Strategy: spread
Filters: health, port, containerslots, dependency, affinity, constraint
Nodes: 1
 localhost.localdomain: 127.0.0.1:2375
  └ ID: ZUIV:BMPV:3B5R:2WBC:JXEI:2S6H:XM3H:66W5:UZQI:NJON:JY4T:HIFB
  └ Status: Healthy
  └ Containers: 0 (0 Running, 0 Paused, 0 Stopped)
  └ Reserved CPUs: 0 / 8
  └ Reserved Memory: 0 B / 12.1 GiB
  └ Labels: executiondriver=native-0.2, kernelversion=3.10.0-327.el7.x86_64, operatingsystem=Red Hat Network, storagedriver=devicemapper
  └ UpdatedAt: 2016-06-08T04:58:05Z
  └ ServerVersion: 1.9.1
Kernel Version: 3.10.0-327.el7.x86_64
......

Or run a container:

# docker -H tcp://127.0.0.1:3375 run hello-world

Hello from Docker.
This message shows that your installation appears to be working correctly.

To generate this message, Docker took the following steps:
 1. The Docker client contacted the Docker daemon.
......

Enjoy Docker Swarm now!

Reference:
Swarm docs;
Docker Swarm Tutorial and Examples.