How Do You Troubleshoot “Segmentation fault” Errors in Hadoop/HDFS?

Problem scenario
You get a "Segmentation fault" or "Segmentation fault (core dumped)" error when you run any "hdfs" command. (This is open source Hadoop and not a proprietary, or vendor's, version.) What should you do?

Solution

Root cause
There is probably an infinite loop/recursion problem. (Segmentation faults involve writing to the stack. Eventually the memory gets filled up.) There is some configuration problem with your Hadoop / hdfs installation. If the installation used to work, look at backup configurations in Git. If it never worked, this problem will not be easy to correct/fix.

Possible solution
Reinstall Hadoop. You may want to see this posting for assistance.

What Is the Difference between aquota.user and quota.user?

Problem scenario
You found some empty files called aquota.group or quota.group. What is the difference?

Solution
The aquota.user file is for version 2 quota systems (per the man page for edquota). quota.user is for version 1. See also What Is the Difference between Version 1 Quotas and Versions 2 Quotas?

For Databases Is a B-Tree the Same as an Index?

Question
In the context of databases, Is a B-tree the same as an index?

Answer
For many intents and purposes they are the same.

"B-Tree is a self-balancing search tree." Taken from
https://www.geeksforgeeks.org/introduction-of-b-tree-2/

"B-tree is a data structure that store data in its node in sorted order."
https://dzone.com/articles/database-btree-indexing-in-sqlite

This source says that the "B" stands for "balanced" (not "binary"): https://use-the-index-luke.com/sql/anatomy/the-tree

To learn more see this: https://www.geeksforgeeks.org/difference-between-binary-tree-and-b-tree/

How Do You Troubleshoot “fatal error: 1/hash.h: No such file or directory”?

Problem scenario
You are trying to install Darling. You get this error: "./include/linux/uio.h:16:31: fatal error: 1/hash.h: No such file or directory
#include RH_KABI_HIDE_INCLUDE()"

What should you do?

Solution
Run this: 

sudo uname -r

Is the kernel version 5 or higher? If not, that is the root cause.

The source of this was https://github.com/darlinghq/darling/issues/975

With Nginx Can a .pem file replace a .crt and/or a .key File?

Problem scenario
You know .crt and .key files can pair as ssl_certificates and SSL certificate keys respectively. In Nginx, can .pem files replace .crt files? Can .pem files potentially replace .key files? Can a .pem file be a substitute?

Solution
Yes to some extent, for the nginx.conf file, a .pem file can replace a .crt file and potentially a .key file too.

To read more, see these:

How Do You Remove the Battery Cover of a Nokia Cell Phone?

Problem scenario
You want to remove the battery from your Nokia flip phone. There is a hinge, and the phone closes as normal like a clamshell. But you need to restart the phone (e.g., and it will not shut off). How do you remove the battery from a Nokia phone?

Solution
While the phone back cover may seem hard to come off, it appears to have a door for the battery cover:

The rounded corners of the line appears to be a seam between two separate components of the back cover. There is only one-piece to the back cover (not a frame with a small door, but one-whole back cover). This is what it looks like when it is off:

Be aware that the whole back cover of a phone like this (a Nokia 2760 Flip) is one piece. By putting your fingernails between the front and back covers, you may have a better chance at removing the back cover and removing the battery of the phone.

How Do You Troubleshoot the Error “unable to prepare context…no such file or directory”?

Problem scenario
You run "docker-compose build" and you get this:

1 error occurred:
* unable to prepare context: unable to evaluate symlinks in Dockerfile path: lstat /home/jane/dockercompose/webapp/Dockerfile: no such file or directory

What should you do?

Solution
Look at your docker-compose.yml file. Look at the "build" stanza. What directory is it referring to? Is a Dockerfile file in that directory? Does that directory exist?

How Do You Get a Bluetooth Speaker to Work with Your Windows Laptop?

Problem scenario
You turn on your laptop and Bluetooth wireless speaker. Your external speaker is not working. What should you do?

Possible Solution #1
Is the Bluetooth device on?

Possible Solution #2
Go to Control Panel -> Hardware and Sound -> Manage audio devices. Do you see a device that could be the wireless one? Click on it and view the properties.

Possible Solution #3
In the right hand corner, right click on the Bluetooth icon and go to "Show Bluetooth Devices". Go to the Audio section. Find the one you want and click "Connect".

Possible Solution #4
Does another device (such as a cell phone) also use the speaker? Can you turn that other device off.

Possible Solution #5
See this posting: How Do You Troubleshoot Computer Speakers Not Working with Your Windows 10 Computer?

Possible Solution #6
As a last resort (if you have a physical hardware problem) you may want to buy new speakers; you can do so here. You can buy a new laptop here.

How Do You Create a docker.service File on a Linux Server?

One of the following apply:

Problem scenario #1
You run this: "sudo systemctl start docker"
But you get this: "Failed to start docker.service: Unit docker.service not found."

Problem scenario #2
You run this: "sudo systemctl start docker"
But you get this:
"docker.service: Service has no ExecStart=, ExecStop=, or SuccessAction=. Refusing."
or this:
"Failed to start docker.service: Unit docker.service not found."

Problem scenario #3
You run this: sudo systemctl enable docker
But you get this: "Failed to enable unit: Unit file docker.service does not exist."

Solution
Use this posting.

How Can You Test Your /etc/fstab File without Rebooting the Linux Server?

Problem scenario
You want modified the /etc/fstab file. You want to reboot, but you are concerned your server won't work if you accidentally had a typo in the /etc/fstab file. What should you do?

Solution

  1. Backup the /etc/fstab file somewhere (e.g., /etc/bak.fstab.bak).
  2. Run a command like this:
sudo mount -o remount,rw /dev/path/to /path/to/use

The /path/to/use is the path that you will see on your Linux system. The source is the /dev/path/to.

  1. The command will fail if your /etc/fstab file had a big problem. Otherwise you will be safe to reboot the server.