LVM Commands and Device Mapper
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Video Transcription
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>> Hello Cybrarians.
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Welcome back to the Linux plus course here at Cybrary.
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I'm your instructor Rob Gills
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and in today's lesson we're going to
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be discussing LVM commands and device mapper.
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Upon completion of today's lesson,
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you'll be able to use the LVM commands
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that we talked about in the previous lesson,
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we're also going to look at dev mapper content,
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the device mapper content.
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Let's get going with some demo time.
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Here we are in our CentOS environment.
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Full disclosure I did a little bit of
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clean up since we were last over here.
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If we do an ls on dev and
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>> then we just look for sd star,
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>> we use the [inaudible] the asterisk.
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We can see that we no longer have
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the sdb1 and sdb2 partitions.
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We just have dev sdb,
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and I've also added another disk device, dev sdc.
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We're going to use the two devices in
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order to create an lvm, a logical volume.
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Let's get started with creating physical volumes.
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We'll do this with sudo and in fact,
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let's just become root using sudo.
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We'll do sudo bash.
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The reason for this is because we have to constantly
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remember to type this command when
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we're working with disks,
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and for the purposes of expediency
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and minimizing errors I'm just going to become root.
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Normally I would recommend that you use sudo,
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but in this case we're just going to become root.
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The first thing we're going to do
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>> is we're going to type
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>> pv create and we're going to
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specify that we want to use dev sdb and
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dev sdc as the two devices
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that we're going to add into our physical volume.
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Once we've done that, we can see that these were
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successfully created as physical volumes.
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We can see this if we run the pv display command.
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That will display what happened with these two devices.
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We can see that dev sdb is a new physical volume
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of eight gigs and the same thing is true of dev sdc.
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We see right here dev sdc is
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a physical volume of eight gigs and so is dev sdb.
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Now what we need to do in order
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to create our logical volume,
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the next step we need to do is to
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>> create a volume group,
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>> and we do that using the vg create command.
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Now we're going to give this a name vg1,
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and we're going to add the two devices,
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dev sdb and dev sdc,
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which are actually physical volumes at this point.
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We can add these in and
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hit "Enter" and now we'll see that
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the volume group BG1 is successfully created.
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Now in order to display this,
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we can run the vg display command.
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Are you getting a sense of the rhythm here?
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Pv create, pv display,
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vg create, vg display,
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and so on and so forth.
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Now if we hit "Enter", we can see that we
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have a volume group, vg1.
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It's a format of lvm2, rewrite,
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and it is 15.99 gigs in size.
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It's just a hair under the two eight gigs,
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so it would be 16 gigs total.
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That's just because of formatting overhead
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and creating it into a volume group.
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Now what we can do that we have
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our physical volume setup as number one and number two,
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we have our volume group setup,
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we can create the logical volume
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using the lv create command.
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The way that we do this is we type lv create and we
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give it a name with the dash n option,
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and we're going to call this lv1.
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We're going to use the dash capital
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L option to specify the size,
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we'll make this a one gig partition,
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and we're also going to need to specify
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the volume group that we're creating
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the logical volume from.
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The very last thing we need to specify is
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our volume group name, which is vg1.
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Then if we hit "Enter", we can see that
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the logical volume lv1 has been created.
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Then just like we saw with pv display and vg display,
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we can also run an lv display.
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That will display the output about
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the information related to what we
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just created and we see that this is in dev vg lv1.
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The lv name is lv1 is associated with
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a volume group vg1 and so on and so forth.
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Now, let's take a look at how this is
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actually set up in device mapper.
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Let's do an ls dash al on dev vg1,
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lv1, which is what we see right up here, the lv path.
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If we hit "Enter", we see that this is actually
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a pointer to dm0,
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and dm0 is the lvm entry for the first logical volume.
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If we were to do an ls al on dev mapper,
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we can also see that vglv1 points to dm0.
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If we are actually to go up to that directory,
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we can see the dm0 directory.
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In dev dm star we'll see dm0.
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Really all that these lvm devices set up in
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dev mapper do is point to another device file,
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the dm0 device file.
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Guess what, if we were to go and run
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an lv create command,
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and let's just say we want to
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create a second logical volume,
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we'll call it lv2,
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what do you think happens?
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Well, what happens is we actually end up with
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a second logical volume and its name will be dm1,
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so dm0 and dm1.
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That's how you work with logical volumes in
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device mapper and how they all get linked together.
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Now next we're going to move on to
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creating ray devices using mdadm.
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However, before we do this we need to
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remove lvms we created.
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Well, how do you think that we do this?
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Well, just like we saw create and display,
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we also have remove commands that are
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used to remove logical volumes,
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volume groups, and physical volumes.
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We can run lv remove.
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We're going to specify first the volume group,
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volume group one,
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>> and then we'll specify logical volume two.
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>> That'll remove that regular active logical volume.
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Now we can do the same thing.
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I just hit the up arrow by
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the way to get back my history.
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I went back to the previous history,
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I removed lv2 and I just put
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in one and we can remove lv1 as well.
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Now that we've removed those,
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we need to remove our volume group.
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Well, we do that with a vg remove command,
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so vg remove vg one.
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That is successfully removed as well.
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Now the last thing we need to do is remove
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the physical volumes because we're going to
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reuse these devices to create our raid.
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We're going to remove dev sdb and dev sdc.
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Pv remove dev sdb and dev sdc and hit "Enter".
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Now we can see that the labels on
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the physical volumes have been successfully wiped,
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they're ready and able to be used again.
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Let's take a look at what we see though in dev mapper.
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If we go back into dev mapper,
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>> what do you think we see?
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>> Well, we see basically absolutely nothing.
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Our dm devices,
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our device mapper devices,
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dm0, dm1 are gone.
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If we go up to our dev directory,
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so you do an ls al on dev dm star,
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>> we don't see anything.
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>> There's no such file in
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our directory, it's been removed.
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That's how we know we're clear and
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ready to move forward with our next lesson.
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We've reached the end of today's lesson.
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In this lesson, we covered creating,
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displaying, and removing logical volumes.
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We also looked at the resulting dev mapper files
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that are created when you set up a logical volume.
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But that's it for this lesson.
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Thank you so much for being here and I
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look forward to seeing you in the next lesson.
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