Wednesday, July 27, 2016

Lesson Learned On Logical Volume Management and RAID

The lesson is this; that just because you can doesn't mean you should.

I wiped off my RAID array and the data in it. It has around 300+G there, mostly downloaded, recoverable, and back-up'd to a CD/DVD content. I was trying to employ Logical Volume Management or LVM on an existing RAID array. After tackling RAID these couple of months, I figured I'm ready to face another monster. LVM is awesome because it can do partitioning, resizing, formatting, creating snapshots, while the system is online. You do not have to take it offline which to me is like magic.

Now that the data is gone, there really is no reason not to LVM the RAID. So after reinstalling my operating system (which is another long story), I reassemble RAID with:

#mdadm --assemble /dev/md0  /dev/sda  /dev/sdb  /dev/sdc

and checked the status of the array with:

#cat /proc/mdstat
#mdadm --detail /dev/md0

I checked the status of all the logical volumes with any of the following commands:

#pvdisplay     ;to check the physical volumes
#vgdisplay     ;to check the volume group
#lvdisplay      ;to check the logical volumes

I know that my home directory is going to need a lot of room for music and videos. I will have to create logical volumes for it to utilize the RAID which is now empty. I would leave some free space there for some flexibility.

To create logical volumes, I plan to extend the existing volume group first to include the newly assembled RAID. So I typed:

#vgextend ubuntu-vg /dev/md0

Then to create the logical volumes I need I typed:

#lvcreate -L size -n Musiclv ubuntu-vg
#mkfs.ext4 /dev/mapper/ubuntu-vg/Musiclv
#lvcreate -L size -n Videoslv ubuntu-vg
#mkfs.ext4 /dev/mapper/ubuntu-vg/Videoslv

I tested mounting them to home directory. I tried:

#mount /dev/mapper/ubuntu-vg/Musiclv /home/user/Music   ;since the Music folder already exist, I don't have to create them.
#mount /dev/mapper/ubuntu-vg/Videoslv /home/user/Videos     ;since the Videos folder already exist, I don't have to create them.

To check if they work, open the file manager, nautilus, and check out properties in particular. If you have trouble with permissions and ownerships of the files, then you should try:

#chmod 777 /home/user/Music
#chown user:user /home/user/Music
#chmod 777 /home/user/Videos
#chown user:user /home/user/Videos

These last series of commands will let you read and write your files and execute operations on them.

To make these changes more permanent so that even if you end your session and reboot they will stay, then we have to edit /etc/fstab file.

#gedit /etc/fstab

Put entries for each lv you created, something like:

    filesystem       mount point        type         options        dump        pass

/dev/mapper/ubuntu-vg/Musiclv    /home/user/Music     ext4     defaults     0       0
/dev/mapper/ubuntu-vg/Videoslv   /home/user/Videos     ext4     defaults     0       0

As the names of the logical volumes imply, I plan to put music and videos on these folders.

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