Linux Logical Volume Manager (LVM) on Software RAID
Logical Volume Manager is now included with most Linux
distributions. The RedHat 8.0 installer even allows you to create
LVM volumes during initial install. LVM offers capabilities
previously only found in expensive products like Veritas.
If you plan on using LVM, I really recommend doing so on a
RAID system, either hardware or software. Disks are dirt cheap
nowadays, so there's really no excuse not to have mirroring (or
RAID 5 if that's better for your usage). The RedHat installer will
let you create LVM volumes on top of RAID volumes (it's a bit
confusing, but it works), or you can do it later. Also remember
that raid is not a substitute for regular and reliable backup!
Before we get to how you'd do this, let's go over what LVM gives
you. First, LVM combines physical devices (partitions on disks in
this case) into what it calls Volume Groups. Filesystems are then
built on the Volume Group or groups. You can have a Volume Group
that has only one disk partition in it, or several partitions on
one or more disks. Each Volume Group can contain multiple
filesystems. With this, you gain a lot of new capabilities.
- Increase or decrease filesystem size
Decreasing the size of a filesystem returns space to the Volume
Group. Increasing draws space from it. If you have too much space
allocated for a file system, you can decrease it, and use that
space somewhere else that needs it.
Yes, you can do that with "parted", but it's not at all the
same. You aren't dealing with contiguous disk blocks with LVM - all
you need is free space in the Volume Group.
- Add more physical storage
You can add more physical drives to an existing Volume Group,
which of course immediately gives you more room to extend file
- Create "snapshots" of filesystems.
This is a great feature, but it requires a bit of explanation,
so we'll leave the details for later. The use of snapshots is to
freeze a filesystem in time, and let you go on using it while you
leisurely back up the frozen data. What's wonderful about this is
that it is NOT necessarily a full copy of your original file system
- more on that later.
- Move Volume Groups to new physical storage
I'm not going to cover this here, but it's a great feature, and
you can even do it while the filesystems are in use!
I'm not going to cover that here (there are some references at
the bottom of this article that do cover it), but you can get the
performance benefit of striping as part of LVM. If you do this, it
is even more critical that you mirror your drives. If you used RAID
5, there wouldn't be any point in striping in LVM as you already
would have that benefit (see Raid
if you don't understand that).
Configuring LVM on mirrored drives during RedHat Install
You might want to review Software Mirroring if you have never
done that. Software mirroring is where we start by creating raid
devices upon which we will create LVM volumes. For this test
machine, I created a layout like this:
||102 MB primary partition
||(rest of disk)
||102 MB primary partition
||(rest of disk)
Note that there are two swap partitions, and that they will not
I then made two raid devices, choosing "Physical Volume (LVM)"
as the filesystem type for the second.
||/dev/hda1 and /dev/hdb1
||/dev/hda3 and /dev/hdb3
||Physical Volume (LVM)
Let's stop for a second and go over this carefully because it
can be confusing. We've created mirrors of our boot partition and
what would ordinarily become root. But there is no root partion
yet. The place where it will go is mirrored, but we aren't creating
a file system just yet. The next step is to click LVM to create a
Volume Group. A volume group needs one or more Physical Volumes,
and we have one. Note that is very different from Raid where you
need at least two Software Raid partitions to do anything. For a
LVM volume group, one physical volume is enough. It is here, in the
Volume Group, that we create filesystems. Note that is plural. With
raid, your partition is one filesystem, but with LVM you can create
multiple filesystems within one Volume Group. To do that, just
click the LVM button and it will bring up a new Volume Group. As we
only have one Physical Volume here, that's what it will use, and
it's ready to let you add file systems.
Note: although I put / into the LVM for this test setup, it
may not be a good idea to do so because emergency boot media
probably won't understand LVM filesystems. Supposedly RedHat
8.0 does support / on LVM (but not /boot).
So I added two filesystems, each 4GB in size, and had one mount
at / and the other at /var. I had much more space available (this
was a 30GB drive), but I didn't bother to use it at this time. I
then proceeded with the install as usual. Well, almost as usual.
Several times in testing I had the problem of not getting focus
when it was time to put in the root password. The solution is to
right click over the field, choose "Select All" and then click in
the field again.
Extending File Systems
After booting up, I had the expected 4GB / and 4GB /var. As you
will remember, the actual disk is much larger, so let's add more
filesystems to it:
lvcreate -L 4G -n mylv Volume00
mkfs -t ext3 /dev/Volume00/mylv
mount /dev/Volume00/mylv /mylv
That added a 4GB filesystem. Unfortunately, I made a mistake - I
needed 6 GB. Well, at this point I could just blow it away
(lvremove /dev/Volume00/mylv), but suppose I'd already added a lot
of data? No problem, just unmount it and extend it:
e2fsadm -L+2G /dev/Volume00/mylv
mount /dev/Volume00/mylv /mylv
The "ef2fsadm" actually runs lower level commands for you: it
fscks the file system, runs lvextend to extend the logical volume,
and then resizes the filesystem to fit. Much easier to let e2fsadm
handle the whole job.
Adding more disks
If I really needed a lot more space, I could add a hard drive
and use "pvcreate" to add it as a physical volume, and then
"vgextend" picks up that space:
vgextend Volume00 /dev/sda1
It just can't get much easier! Remember though: those disks
should be mirrored too.
This is a very useful feature. Many of us have the situation
where important data needs to be backed up, but it cannot be used
while the backup is running because then the backed up files would
be out of sync with each other. For example, you have an accounting
system that is recording orders. The accounts receivable file gets
backed up now, and you take an order. Both a/r and the customer
file get updated to reflect the new order, but a/r has already been
backed up. When the customer file finally makes it to tape, it's
not consistent with a/r, and of course it needs to be. Without
snapshots, your only recourse is to stop taking orders while the
backup runs. If you have lots of disk space, you could copy the
whole accounting system and backup the copy, but that can take a
lot of time too, and you may not have the space. Snapshots are the
solution. Before you do the next step, make sure you've put a few
files in /little, and make at least one of them unimportant. Then
create the snapshot.
lvcreate --size 200M --snapshot -n mysnap /dev/Volume00/mylv
mount /dev/Volume00/mysnap /mylvsnap
Right off the bat you should have noticed something strange. We
created mysnap very specifically with a size of 200MB, and trust
me, that's all it took away from us, but df shows it being the same
size (6GB) as mylv. We'll get back to why this is in a minute, but
first take a look at the files in /snap. They are identical to the
files in /mylv, right? OK, now go edit a file in /mylv. Does it
change in /snap? No, it does not. Remove a file in /mylv - it's
still there in /snap. Add a new file to /mylv, and that does NOT
appear in /snap. How is this done, and most especially how is it
done in 200MB?
It's not magic
OK, it is magic. What is going on is that /snap contains
absolutely nothing UNLESS something changes back at /mylv. If you
ask for a file from /snap that has not changed, the data is read
right from /mylv. But if a file IS changed, before the change is
written, the data blocks that don't yet have the changes are
written to /snap. Note that entire files are NOT written, just data
blocks that are about to change. So, as long as we don't change
more than 200MB worth of data in /mylv, we can have our cake and
eat it too. Our procedure will be:
- Stop using the filesystem, shut down any databases that need to
be shutdown etc.
- Create the snapshot
- Start up our databases, go back to work.
- Start backing up /snap
Our time without access is minutes or seconds - just however
long it takes to stop the processes and restart them, basically.
The backup can take its sweet time. Well it can if it doesn't take
so long that we need more than 200 MB to store our data that is
changing. That does mean that the size of mysnap does have to be a
bit of an educated guess. It also means that as soon as you are
done with the backup, mysnap should be removed:
If you don't remove it, it will go on copying data as it is
changed and eventually it will run out of room. You can't just
leave it there for next time!
Some other helpful links:
articles, comments, book reviews or opinions here!
© November 2002 Tony Lawrence All rights
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