linux

Replacing a degraded drive in a software RAID 1 array

After physically replacing the degraded drive with the new one and rebooting the system, run the folowing command to create exactly the same partitioning on /dev/sdb (new drive) as on /dev/sda (healthy existing drive):

sfdisk -d /dev/sda | sfdisk /dev/sdb

You can run

fdisk -l

to check if both hard drives have the same partitioning now.

Next we add /dev/sdb1 to /dev/md126 and /dev/sdb2 to /dev/md127:

mdadm --manage /dev/md126 --add /dev/sdb1
mdadm --manage /dev/md127 --add /dev/sdb2

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How to investigate sessions

To list all current sessions:

loginctl list-sessions

To investigate details of particular session run:

loginctl show-session [session_id]

The sessions files are located in /run/systemd/sessions on Fedora-based systems and in /run/systemd/sessions on Debian-based systems.

If sessions fill up the /run directory for 100% then per recommendation on
https://www.centos.org/forums/viewtopic.php?f=48&t=65472:

How to check when a file was created on a Linux system

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Recursively copying between two directories including hidden files

Sometimes we need to override large structure of directories and subdirectories with newer files.

There are two ways of copying all the files in one directory including subdirectories and hidden files to another directory:

cd /orig/dir
tar cvf - . | (cd /dest/dir; tar xvf -)

which tars up the current directory to stdout then pipes it to a subshell that first cd's to the destination directory before untarring stdin.

The second way of doing this is using cp:

yes | cp -rT /orig/dir /dest/dir 2> /dev/null

Creating a patch file with diff command

To create a patch file simply run:

diff -u functions.php.orig functions.php > a.patch

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Removing the RAID metadata

If you see the “Disk contains BIOS metadata, but is not part of any recognized BIOS RAID sets. Ignoring disk sda” error message during OS installation, then you need to remove the raid metadata the “classic” way:

dmraid -r -E /dev/sda
Do you really want to erase "pdc" ondisk metadata on /dev/sda ? [y/n] :
y

How to keep a detailed audit trail of what’s being done on your Linux systems

Intrusions can take place from both authorized (insiders) and unauthorized (outsiders) users. My personal experience shows that unhappy user can damage the system, especially when they have a shell access. Some users are little smart and removes history file (such as ~/.bash_history) but you can monitor all user executed commands.It is recommended that you log user activity using process accounting. Process accounting allows you to view every command executed by a user including CPU and memory time.

Migrate files and directories between two remote servers with rsync and progress bar

To push an archive file from local to remote server with progress bar use:


rsync --rsh -av -e 'ssh -p #####' --progress --partial file.tgz root@xxx.xxx.xx.xxx:/backup

To push the content of the directory from local to remote server recursively with progress bar use:


rsync -avz -e 'ssh -p #####' --progress --partial localdirectory/ root@xxx.xxx.xx.xxx:/backup/

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Partition X does not end on cylinder boundary

While reviewing the partion layout on one of my hard drives, I noticed a number of “Partition X does not end on cylinder boundary” messages in the fdisk output:

$ fdisk /dev/sda
Command (m for help): p

Disk /dev/sda: 80.0 GB, 80000000000 bytes
255 heads, 63 sectors/track, 9726 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0xac42ac42

Device Boot Start End Blocks Id System
/dev/sda1 * 1 26 204800 83 Linux
Partition 1 does not end on cylinder boundary.

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Monit configuration to monitor Memcached instances

Monit is a free open source utility for managing and monitoring, processes, files, directories and filesystems on a UNIX system. It is pretty easy to configure and even easier to use. It comes with a simple web server to monitor statuses of the alerts you set (with basic HTTP authentication). On Monit W’k’ you can find configuration examples for different services. Just in case you need a configuration example for Memcached instances, here is what you need.

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