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[No audio]

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Filesystem corrupted or if there

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is a filesystem corruption.

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Well, what is a filesystem?

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Filesystem consists of the directories or folders

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that's you create in your operating system.

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So there are different type of filesystems.

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The filesystems are like ext3, Linux

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ext4, xfs, NTFS or Fat and Windows.

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These are all different type of file

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systems that we have in operating system.

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A filesystem layout and partitions

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are like /var, /etc, /root, /home.

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So whenever you carve a disk and you give that disk

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to your operating system, the first thing you have to define

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is what type of filesystem do you want for it?

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Then you pick a filesystem, for example, ext4.

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Once that is assigned to your filesystem, the

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next step is to partition that disk or that

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filesystem in the way you want.

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If you leave it a default file

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system, then everything goes under slash.

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But if you wanted to partition with advanced partitioning

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option while you're doing the installation, then you could

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use /var, /etc, or whichever the partition or

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mount point that you want to specify.

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So what happened is when the filesystem is corrupted,

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it usually gets corrupted in one of these partitions.

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Maybe the /var is bad, /etc, or /root is bad.

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That's why sometimes the system administrators, the

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way they design the operating system is

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they slice each partition differently.

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So this way if there is a problem

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with var, it only stays with var.

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It doesn't impact the whole slash filesystem.

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So that is one of the main reason people

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pick the partitioning of your disk instead of just

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picking one big large partition which is slash.

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Now, how do we check the filesystem?

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Well, the first command that you

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need to run is df command.

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We already talked about it.

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df command tells you what type of file

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system you have and where that filesystem

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is being partitioned and where is it mounted.

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Also you could run the command fdisk with minus

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l option to see all the disks that you have

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in your system and also each partition of that disk.

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If your system is running on lvm, then

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you could run many lvm commands like

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lvdisplay, pvdisplay, the lvblock.

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So there are many commands that you

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can run to see your filesystem.

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Now, let's get into troubleshooting steps.

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So you know your system is running slow

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or you have identified that you have a

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filesystem corruption in one of your partitions.

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So what do you do?

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You first check the /var/log/messages to

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confirm that it is actually the filesystem.

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And you will see a lot of error messages that will

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come up in your /var/log/messages that will say that bad

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log on this sector, on this disk and you need to

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fix it or you need to run fsck,

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you will see that in your /var/log/messages.

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So /var/log/messages or any type of log

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like /var/log/syslog are very important in a

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system structure to identify where the problem is.

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Then once you know that you do have

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a filesystem problem or a corruption and

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you identified which partition that filesystem is

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corrupted, then you run the command fsck.

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This is the command fsck,

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which stands for filesystem check.

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This command has been used with many different Linux

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as well as Solaris or Unix flavors as well.

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It's still there, it works perfectly.

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There are some of the new commands that

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are in there for different filesystem.

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Maybe XFS filesystem will run xfs_repair,

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which we'll talk about in a little bit.

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So anyway, we run the fsck on

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block device which is /dev/sda.

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We will not run on the mount point, meaning

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these are the mount points to /var, we cannot run fsck

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on /var. We have to run on the actual mount

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where it's coming from, the source.

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That would be the block device.

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Then make sure we are unmounting the

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filesystem before we are running fsck.

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Because if you run fsck on a mounted filesystem or

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mounted partition that is already mounted on /var, /etc, it

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will give you an error message and it will say you

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cannot mount it because that filesystem is busy.

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That is why one of the things that a lot

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of people get into trouble saying when they are trying

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to do the corruption cleanup of the corruption filesystem

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and they run fsck, and it doesn't run.

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And then they're sitting in trying to find out why it

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is not running, read the error message, what it says and

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it will tell you why it's not running because it cannot

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run that command on the mounted filesystem.

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So let's get into our Linux machine.

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I have already opened a terminal to my Linux machine

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and the first thing we will see is, we'll see

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the layout of our filesystem, and for that we'll

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run the command df -h. h is for human readable option.

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You could pick any option you want.

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If you pick, I suppose t, that will also

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tell you what type of filesystem it is.

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It is xfs, tmp filesystem, or exe filesystem.

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So let's just run df -h, and you will see there

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are different filesystems are also there as tmpfs.

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These are just tmporarily, this is

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not the actual filesystem.

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So what you could do is you could do df

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-h and grep -v to exclude tmpfs.

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So now you know exactly how many partitions you have.

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You have two partitions.

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One is mounted on slash, which is just slash

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on the root level and the second one is

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/dev/sda1, which is mounted for boot.

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boot is usually mapped on a regular block device.

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It does not get mapped on the lvm.

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So up here, you'll see this is the lvm.

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It always starts with /dev/mapper.

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So we identified it.

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Now, in your system, if you are working in a

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corporate environment or any of your system,

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you're probably going to see,

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more mounted points.

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You probably have /var, /etc, /opt,

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/sbin, or /tmp, whatever partition you have,

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it's ok. I'm not going to go into everyone of them

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But I'm just saying, you're probably going

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to see a lot more mount points on your system.

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If you don't, that is fine.

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But let's go, come in here,

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and to cover this, I have actually

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attached a disk through my virtual machine.

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And I am going to mount that disk first

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just to show you how we could run fsdk.

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So that disk I will do df -k, fdisk -k.

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Oh sorry, fdisk -l,

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and here, if you notice, I have a

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new disk, which I have already partitioned.

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I just wanted to add that I

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have already partitioned that new disk.

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If you notice right here, I have

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a disk, /dev/sdb, which is five gig.

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This is what I got from my virtual

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machine, and I have already partitioned that.

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Now I have to mount that somewhere.

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So I will mount it in one of

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the directory that I have created earlier.

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So that directory is in my slash directory,

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which is I named it sparedata.

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So I'm going to mount that mount

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/dev/sdb1 to /sparedata/.

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Okay, now it is mounted.

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Now, when I run df -h, of course

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I will grep everything, all the temporary filesystem.

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And now you will see I have basically two filesystems.

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One is sda, and one is sdb.

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They are the same filesystem,

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XFS. I wanted to know, I have discovered that this

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filesystem or this partition point is corrupted.

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Now, I need to fix that or clean

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any bad sectors that is on this partition.

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So I will run the command fsck space /sparedata.

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Now, the reason I'm putting that here, because I want you

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to know that it will not work, because I am trying

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to run fsck on the mount point, which does not work.

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And it will tell you it's a

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directory while trying to open this.

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So we don't do on the directory, we do

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on the block device, as I mentioned before.

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So I will do fsck /dev/sdb1.

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Now I'm running this, it will still fail. Why?

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Because as I mentioned in my last part, it

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says unmount filesystem and then run fsck.

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Unmounting means it is already

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mounted on this directory.

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So that's why it's not going to work.

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But I will go ahead and run

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it to see the error message.

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Okay, so when I ran it, it says fsck from utility-linux so

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and so, and the version and the date, and it says

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/dev/sdb1 is mounted. e2fsck: Cannot continue.

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So aborting.

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Basically, this e2fsck is a newer command.

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But for fsck, I'm sure fsck probably have a link to this.

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This is the new command that you could run. Anyway,

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we could talk about new commands later.

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But right now our focus is to find out

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why we cannot run fsck on /dev/sdb1.

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And we know we cannot, because it is mounted.

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So what do I have to do?

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I have to unmount it.

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[No audio]

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umount, the command to unmount is umount /sparedata.

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That's the partition or that's the mount point I have.

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So I'll run that command and now it is unmounted.

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I will do df -h again

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and you'll see it is not there.

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Now I could run fsck /dev/sdb1 by hitting up arrow keys

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which will repeat all the commands that we ran earlier,

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I hit Enter. As soon as I hit enter,

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it says fscl from utility so and so, and version, date,

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it says /dev/sdb1, and clean.

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And it has some calculations and the files, it's telling you

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how many files it has, how many blocks it has, since

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this is only five gig and it went pretty fast and

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there is no files or data on that spare disc right

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now, that is why it went pretty fast and you see

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you got the result right away.

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If you have that, let's say 100 gig of disk and has it so

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much data on it, then it could take a long time to run.

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It will go into it and it will check everything that it has

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to check on for the bad blocks and it will fix it.

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So it's also possible while it is fixing it, it

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will ask you, hey, I found the fix for this,

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do you want me to fix that bad block?

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And you will have to enter y,

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to say yes, go ahead.

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So one thing is you could keep saying y, y,

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and keep entering in on the prompt, or you

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could just simply say fsck -y.

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It means whatever that you're going to ask me,

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my answer is yes.

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So then you hit Enter and it will go through

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all the filesystem again and it will clean up.

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Now, next thing I wanted to talk about, which

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is one of the biggest thing a lot of

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people experience is they say, okay, my filesystem

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is corrupted, but it is root filesystem.

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Now how do I clean that when it is mounted?

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If I unmount it, it's not going to let me unmount

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because that's where I am running my entire operating system.

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So how can I do that?

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Well, that's a very good question if

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that is coming in your mind.

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Many times we have that issues we cannot clean up.

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So the only way we could do that is we have to

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boot our machine through the CD ROM, we have to mount our

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CentOS or Red Hat cd to our Linux machine right here and

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we have to boot from that CD ROM and then once it

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boots, it will give us the rescue option.

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We'll pick the rescue option and then we can

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fix this issue if there are any bad sectors

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on the root or any partition that cannot be

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mounted while the operating system is running.

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Now, how can I do that?

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So for that, you cannot use a PuTTY session for that.

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You have to actually console into the machine

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because you will be rebooting the machine and

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you will be bringing down all the network

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connections and all the multi user environment.

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So I will go into my Linux machine through console.

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Okay. I am logged into console.

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Now I have to attach the device,

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sorry, the ISO image of my CentOS.

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So go to Device, Optical Drives, and choose a disk image.

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Once I go in here, it will tell me where the file is.

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I'll say Desktop, it is on my Desktop,

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and the file is CentOS, click Open.

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If you want to confirm it has attached,

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you could go back to Devices, Optical Drives, and

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you see the check mark is right here.

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So it means it is mounted.

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So now all I have to do is do a reboot and hit Enter.

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[No audio]

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When it reboots, you see in the back I have the

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PuTTY session and it says it lost the server unexpectedly.

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That's fine.

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Now I am at this section

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where it says Install CentOS 7.

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You probably have seen that if

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you have recently done the installation.

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So Install CentOS is perfect.

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You don't want us to do that because this

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is if you are doing the install from scratch,

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Test this media & install CentOS.

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It's the same thing, but it will test it again.

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Our option is Troubleshooting.

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I'll go in and hit Enter in Troubleshooting.

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Now the next option is Install CentOS in

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basic graphical mode.

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Again, we are not installing, we are doing Rescue.

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So come down to the selection to the

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option where it says Rescue ascent OS system.

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If you are using Redhat it

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would say rescue a Redhat system.

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So hit Enter.

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[No audio]

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Now it's basically starting up the Operating System.

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Not actually the operating system, but the system

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that is actually on the disk that is

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used to go through the installer.

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See, it says starting installer.

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Well, in fact it's not doing the

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installer, so don't be alarmed by that.

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Now here we are in the rescue mode.

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Read it very carefully, says the environment.

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The rescue environment will now attempt

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to find your Linux installation and

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mounted under the directory /mount/sysimage.

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You can then make any changes required to your system.

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Choose 1 to proceed

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With this step you can choose to

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mount your filesystem read-only.

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Instead of read, write by choosing 2.

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If for some reason this process does not work,

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choose 3 to skip directly to a show.

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So if I mounted, if I pick one, then it will

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mount my filesystem which is root to /mnt/sysimage.

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[No audio]

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So suddenly I lost the screen.

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But anyway, so I don't want to mount that.

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If I mounted then I cannot run fsck

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which I will be in the same situation.

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I don't want to mount it as read-only.

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So I will just skip straight to the shell.

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So I will pick option three and hit Enter.

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Now I have the shell, so I run df -h command and

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you will see none of the filesystem will show up here.

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That actually the partitions that we have when the actual

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operating system runs, the one that you see right now,

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it is actually coming straight from the cd.

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So you know that already.

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Your filesystem that you need to

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clean actually lives on /dev/sda.

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So you will run now you run fsck minus

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y, if you want to/dev/sda1,

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because you have to specify the entire partition for

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every partition that you have on the disk.

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Now you hit Enter and it says fsck from

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utility-linux 2.23.2. /sbin/fsck.xfs : XFS file system.

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It means I cannot run fsck for

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the partition that is on that list on

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or that has assignment of XFL filesystem.

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So I have to run the command fsck.xfs.

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Now run /dev/sda1.

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Now if you wish to check the consistency of

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XFL system or repair damaged filesystem, see this.

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Now it's saying now go to this command.

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This is the one I was talking about.

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You will have to run this command instead of fsck.

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So now you run a /dev/sda1, hit Enter and

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it's going through all the phases, phase one to six.

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And it's actually resetting the content, resetting checking the

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blocks and it's verifying and it says verify and

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correct link counts and at the end don't.

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So basically it went through the whole lot.

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If you had a big disk space with a lot of data

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on it, then it will take a long time as I mentioned.

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So now since it's mounted, it's cleaned, your

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corruption is cleared on your filesystem.

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Now you could simply do reboot, but before you

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do the reboot, just unmount your optical drive so

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it won't start off the ISO image.

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Force Unmount and hit Reboot.

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It will reboot and it will come

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back as clean disk and no corruption.

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You will also check in the /var/log/messages.

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Anyway, so this is about

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this lecture filesystem corruption.

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Then you have to follow these steps one by one again.

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If you have any questions, please send me

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a message and I'll be happy to answer

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anything that you haven't in mind.

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[No audio]