[LFCS] Managing Software RAID

mdadm is a super cool command in Linux used to manage MD devices aka Linux Software RAID. Before we jump in, let’s see what is RAID. – –Redunant array of independent disks
–if disk gets corrupted, then data loss
–using RAID, if one disk fails, other will take over

$ man page says this,

RAID devices are virtual devices created from two or more real block devices. This allows multiple devices (typically disk drives or partitions thereof) to be combined into a single device to hold (for example) a single filesystem. Some RAID levels include redundancy and so can survive some degree of device failure.

Understanding RAID soln,
RAID O- Striping { one big device based on multiple disk, no redundancy or easy recovery}
RAID 1- Mirroring { 2 disks, identical }
RAID 5- striping with distributed parity { if data is written with parity info, if one disks fails, then restore the data }
RAID 6- striping with dual distributed parity { redundant parity is written,advancement of RAID 5 }
RAID 10- mirrored and striped { minimum of 10 disks, 2 for striping, 2 for mirrored}

Sample question: How to create a RAID 5 Device using 3 disk device of 1 GB each. Also allocate additional device as spare device.
— Put a file system on it and mount it on /raid
— Fail one of the devices, monitor what is happening
— Replace the failed device with spare device

Solution:

$ cat /proc/partitions
$ fdisk -l { list the partition tables for the device, if no device specified then list all the partitions from the system }
$ fdisk /dev/sdc
-n create a new partitions { size as +1G }
-m {help}
-t :L {enter “fd” for Linux raid auto}
-w { write the entries to persist }
$ partprobe { inform the OS partition table changes }
$ vim /etc/fstab { before we proceed, let’s verify the disks are not used for any mounting. In my case, I had used as swap device mounting so got an error saying device is busy error. Rmv the entry, reboot }
$ mdadm –create /dev/md1 -l 5 -x 1 –raid-disk=3 /dev/sdc1 /dev/sdc2 /dev/sdc3 /dev/sdc4 –verbose –auto=yes
$ mdadm –detail /dev/md1 { list details after creation, should see 3 device + 1 spare device }

$ mdadm —fail dev/md1 /dev/sdc1 { to simulate the failure }
$ mdadm –remove /dev/md1 /dev/sdc1 { remove the faulty one }
$ mdadm –add /dev/md1 /dev/sdc1 { add the device back to the pool as spare device if healthy }

other disk related commands,
$ blkid $ blkid /dev/sdc
$ df -h, df -h -T, df -hT /home
$ du -h /home, du -sh /home/mydir
$ mount /dev/sdc5 /mnt, cd /mnt , touch file1 { after mounting make entry in /etc/fstab to persist}
$ mount -a { to mount all fs mentioned in fstab}
$ mkfs.ext4 /dev/sda4 { format a partition of type ext4, after creating a partition }

Command output:

root@mikky100:~# mdadm –fail /dev/md1 /dev/sdc1 { Simulate the failure }
mdadm: set /dev/sdc1 faulty in /dev/md1

root@mikky100:~# mdadm –detail /dev/md1 { view the detail after the failure, we should see the spare disk getting rebuild }
/dev/md1:
Version : 1.2
Creation Time : Mon Jun 11 06:10:34 2018
Raid Level : raid5
Array Size : 1951744 (1906.32 MiB 1998.59 MB)
Used Dev Size : 975872 (953.16 MiB 999.29 MB)
Raid Devices : 3
Total Devices : 4
Persistence : Superblock is persistent

Update Time : Mon Jun 11 17:06:09 2018
State : clean, degraded, recovering
Active Devices : 2
Working Devices : 3
Failed Devices : 1
Spare Devices : 1

Layout : left-symmetric
Chunk Size : 512K

Rebuild Status : 3% complete

Name : mikky100:1 (local to host mikky100)
UUID : 772f743c:b1209727:6910411d:690d6294
Events : 20

Number Major Minor RaidDevice State
3 8 36 0 spare rebuilding /dev/sdc4
1 8 34 1 active sync /dev/sdc2
4 8 35 2 active sync /dev/sdc3

0 8 33 – faulty /dev/sdc1

root@mikky100:~# mdadm –detail /dev/md1
/dev/md1:
Version : 1.2
Creation Time : Mon Jun 11 06:10:34 2018
Raid Level : raid5
Array Size : 1951744 (1906.32 MiB 1998.59 MB)
Used Dev Size : 975872 (953.16 MiB 999.29 MB)
Raid Devices : 3
Total Devices : 4
Persistence : Superblock is persistent

Update Time : Mon Jun 11 17:08:13 2018
State : clean
Active Devices : 3
Working Devices : 3
Failed Devices : 1
Spare Devices : 0

Layout : left-symmetric
Chunk Size : 512K

Name : mikky100:1 (local to host mikky100)
UUID : 772f743c:b1209727:6910411d:690d6294
Events : 37

Number Major Minor RaidDevice State
3 8 36 0 active sync /dev/sdc4
1 8 34 1 active sync /dev/sdc2
4 8 35 2 active sync /dev/sdc3

0 8 33 – faulty /dev/sdc1

root@mikky100:~# mdadm –add /dev/md1 /dev/sdc1 { add the disk back as spare }

root@mikky100:~# mdadm –detail /dev/md1
/dev/md1:
Version : 1.2
Creation Time : Mon Jun 11 06:10:34 2018
Raid Level : raid5
Array Size : 1951744 (1906.32 MiB 1998.59 MB)
Used Dev Size : 975872 (953.16 MiB 999.29 MB)
Raid Devices : 3
Total Devices : 4
Persistence : Superblock is persistent

Update Time : Mon Jun 11 17:12:21 2018
State : clean
Active Devices : 3
Working Devices : 4
Failed Devices : 0
Spare Devices : 1

Layout : left-symmetric
Chunk Size : 512K

Name : mikky100:1 (local to host mikky100)
UUID : 772f743c:b1209727:6910411d:690d6294
Events : 39

Number Major Minor RaidDevice State
3 8 36 0 active sync /dev/sdc4
1 8 34 1 active sync /dev/sdc2
4 8 35 2 active sync /dev/sdc3

5 8 33 – spare /dev/sdc1

[LFCS] terminal multiplexer commands $ tmux

tmux, htop and bash-completion are the 3 useful commands for the exam. You can install them using $ apt install tmux htop bash-completion.Let’s see the commands for tmux,

Session management:-
$ apt install tmux -y
$ tmux new -s <session-name>
$ tmux attach -t <session-name> {continue to run in the bg if we detach}
$ tmux list-sessions

Session command:-
<Cntl-b> % -> Split the window vertically
<Cntl-b> ” -> Split the window horizontally
<Cntl-b> x -> kill the current pane
<Cntl-b> Up, Down, Right, Left cursor -> switch the cursor from one pane to the other
<Cntl-b> x -> Close the current pane
<Cntl-b> [ -> Scroll within a pane (use q to exit)

[LFCS] Commands to manage and configure containers in Linux

– LXC(Linux container) is an OS level virtualization for running multiple isolated Lx systems (containers) using single kernel
– LXC combines the kernel’s cgroups (Control Groups) to provide isolated space for our application
– Lx kernel provides the cgroups functionality + namespace isolation –> cgroups is the brain behind the virtualization
– cgroups provides { resource limiting, prioritization, accounting and control }
– various projects use cgroups as their basis, including Docker, CoreOS, RH, Hadoop, libvirt, LXC, Open Grid/Grid Engine, Kubernetes, systemd, mesos and mesoshpere
– initial version of Docker had LXC as execution environment, but later replaced with libcontianer written in go lang
– both dockers and LMCTFY taken over the containers space and used by many companies

>>>LXC – Linux container commands

$ sudo -i {switch to root account}
$ apt update
$ free -m { check your memory availability, -m for MB, -G for GB }
$ apt install lxc  { linux container, docker is based on this/type of }
$ systemctl status lxc.service
$ systemctl enable lxc
$ lxc ->tab -> tab  { to see all the lxc- commands }
$ cd /usr/share/lxc/templates
$ ls { should see the list of templates }
$ lxc-create -n mylxcontainer -t ubuntu { should create ubuntu container based on the specified template}
$ lxc-ls { list the local container, ubuntu should appear with name mylxcontainer }
$ lxc-info -n mylxcontainer { should see the status as STOPPED }
$ lxc-start -n mylxcontainer
$ lxc-info -n mylxcontainer { should see the state as RUNNING }
$ lxc-console -n mylxcontainer { console login to the container, username -ubuntu, pass-ubuntu }
$ ubuntu@mylxcontainer:~$ { upon login your console prompt changes takes you to ubuntu }
$ uname -a or hostname { to confirm you are within the container }
$ Type <cntrl+a q> to exit the console
$ lxc-stop -n mylxcontainer
$ lxc-destroy -n mylxcontainer

>>>Docker container commands
$ apt update
$ free -m
$ apt install docker.io
$ systemctl enable docker
$ systemctl start docker
$ systemctl status docker
$ docker info
$ docker version
$ docker run hello-world { to pull the hello-world for testing }
$ docker ps
$ docker ps -la or $ docker ps -a { list all the containers }
$ docker search apache or microsoft { to search container by name }
$ docker images { to list all the images in localhost }
$ docker pull ubuntu
$ docker run -it –rm -p 8080:80 nginx  { for nginx, -it for interative }
$ docker ps -la { list all the containers, look for container_id, type first 3 letters which is enough }
$ docker start container_id or ubuntu { say efe }
$ docker stop efe
$ docker run -it ubuntu bash
$ root@efe34sdsdsds:/# { takes to container bash }
<type cntrl p + cntrl q> to switch back to terminal
$ docker save debian -o mydebian.tar
$ docker load -i mydebian.tar
$ docker export web-container -o xyz.tar
$ docker import xyz.tar
$ docker logs containername or id
$ docker logs -f containername or id { live logs or streaming logs }
$ docker stats
$ docker top container_id
$ docker build -t my-image dockerfiles/ or $ docker build -t aspnet5 .  { there is a dot at the end to pick the local yaml file for the build }

>>>for working with Azure Container

$ az acr login –name myregistry
$ docker login myregistry.azurecr.io -u xxxxxxxx -p myPassword
$ docker pull nginx
$ docker run -it –rm -p 8080:80 nginx { Browse to http://localhost:8080  }
{To stop and remove the container, press Control+C.}
$ docker tag nginx myregistry.azurecr.io/samples/nginx
$ docker push myregistry.azurecr.io/samples/nginx
$ docker pull myregistry.azurecr.io/samples/nginx
$ docker run -it –rm -p 8080:80 myregistry.azurecr.io/samples/nginx
$ docker rmi myregistry.azurecr.io/samples/nginx
$ docker inspect -f “{{ .NetworkSettings.Networks.nat.IPAddress }}” nginx
$ az acr repository delete –name myregistry –repository samples/nginx –tag latest –manifest
$ docker run -d redis (By default, Docker will run a command in the fg. To run in the bg, the option -d needs to be specified.)
$ docker run -d redis:latest
$ docker start $(docker ps -a -q)
$ docker rm -f $(docker ps -a -q)

>>>docker minified version
$ docker pull docker.io/httpd
$ docker images
$ docker run httpd
$ docker ps [-a | -l]
$ docker info
$ docker run httpd
$ curl http://172.17.0.2  <ctrl+c>
$ docker stop httpd
$ docker rmi -f docker.io/httpd
$ systemctl stop docker

Happy learning !

[Linux] LFCS prep and easy way to remember commands

I’m into LFCS certification preparation for the past one year on and off. It is not an easy job for a beginner to digest the information spread across the topics ranging from basic commands to virtualization.

I started as a Newbie to Linux, today having a considerable knowledge on Linux environment but still struggling to use the right switch key at first shot. Very important thing is, we keep forgetting the commands if there is no continuity in preparation especially when not using Linux as primary OS for day work.

One thing I learned during this time period is, “there is always more than one way to do things in Linux”.  That’s the spirit keep motivated me to go strong for the exam. My learning started with deploying 1) Azure Linux VM prepared for RDP, then 2) dedicated laptop installed using bootable USB (thanks to Rufus), 3) then formatted ubuntu and installed Centos to get a feel 4) Again back to Ubuntu

Just delivered a #Azure #Batch demo to Customer $python & #linux machine
Used this code, https://t.co/946q0uZLgZ worked charm #awesome pic.twitter.com/LDnN51364O

— Maheshk (@MahesKBlr) September 22, 2016

5) then Redhat machine in Azure 6) then tried bash on windows (WSL), 7) tried Hyper-V manager to have Ubuntu and Centos but finally heavily using Azure Ubuntu Linux for giving my exam. After all this trial and error, now I can talk about the variants , understand any command and its purpose, can write a small bash script for a cron job, manage system services, install the missing pieces and prepare a machine etc.

What an exiting time to be in tech to use #Linux. This was not an easy as now 15 years ago when fiddling with PCQ Linux #LFCS #Exam #Rocks pic.twitter.com/RzjRUb1fWY

— Maheshk (@MahesKBlr) October 8, 2017

Now, coming back to the topic, It is not always easy to remember and recollect the Linux commands with various switches. It needs a lot of practice and continuous hands on to get memorized otherwise we will be managing with 10-15 commands with lots of trial and error. Now, I love my terminal windows than Linux IDE. I stopped spending time installing GUI or RDPing, I manage to live with only commands for anything. It forced me to think through deep enough and also gives me a various flavor to choose from. Here today, I would like to leave with these 3 Linux tools which would help us any day to get the right commands to use. Later we can deep dive man pages for switches.

tool #1:- Fish is an awesome tool help us to get an autocomplete for the entered command with all possible options. It actually takes to the fish shell, where we need to enter the partially broken commands to get variants.

tool #2:- History command is the one which most frequently use to recollect the command switches. I highly recommend making use of this when we copy paste certain commands from net for the first time to make it work then later we forget the steps we arrived.

 

tool #3:- Apropos, this one is a personal favorite. When I doubt, run apropos command to get all the possible nearest commands. Many times we know there will be a command way to manage database say mysql but won’t know the full command in hand to try. This command fills the gap by providing all the list containing the search keyword.

Few other useful commands for productivity, (1) tmux

Thanks to @tutoriaLinux ; #Linux #tmux tutorial is life safer for exam prep. You can check the man pages & execute the cmds side by side pic.twitter.com/uszwUCQeiH

— Maheshk (@MahesKBlr) September 11, 2017

(2) apt-cache search <pckg name> (3) sysctl (4) find (5) grep (6.) tree (7) watch elinks (8.) ps (9) top/htop (10) tar.

Let me know your favorite ones .