Managing Disk Space with LVM -01

The Linux Logical Volume Manager (LVM) is a mechanism for virtualizing disks. It can create "virtual" disk partitions out of one or more physical hard drives, allowing you to grow, shrink, or move those partitions from drive to drive as your needs change. It also allows you to create larger partitions than you could achieve with a single drive.

Traditional uses of LVM have included databases and company file servers, but even home users may want large partitions for music or video collections, or for storing online backups. LVM and RAID 1 can also be convenient ways to gain redundancy without sacrificing flexibility.

This article looks first at a basic file server, then explains some variations on that theme, including adding redundancy with RAID 1 and some things to consider when using LVM for desktop machines.

LVM Basics

To use LVM, you must understand several elements. First are the regular physical hard drives attached to the computer. The disk space on these devices is chopped up into partitions. Finally, a filesystem is written directly to a partition. By comparison, in LVM, Volume Groups (VGs) are split up into logical volumes (LVs), where the filesystems ultimately reside (Figure 1).
Each VG is made up of a pool of Physical Volumes (PVs). You can extend (or reduce) the size of a Volume Group by adding or removing as many PVs as you wish, provided there are enough PVs remaining to store the contents of all the allocated LVs. As long as there is available space in the VG, you can also grow and shrink the size of your LVs at will (although most filesystems don't like to shrink).

Figure 1. An example LVM layout (Click to view larger image)

Example: A Basic File Server

A simple, practical example of LVM use is a traditional file server, which provides centralized backup, storage space for media files, and shared file space for several family members' computers. Flexibility is a key requirement; who knows what storage challenges next year's technology will bring?
For example, suppose your requirements are:

400G  - Large media file storage
 50G  - Online backups of two laptops and three desktops (10G each)
 10G  - Shared files

Ultimately, these requirements may increase a great deal over the next year or two, but exactly how much and which partition will grow the most are still unknown.

Disk Hardware

Traditionally, a file server uses SCSI disks, but today SATA disks offer an attractive combination of speed and low cost. At the time of this writing, 250 GB SATA drives are commonly available for around $100; for a terabyte, the cost is around $400.
SATA drives are not named like ATA drives (hda, hdb), but like SCSI (sda, sdb). Once the system has booted with SATA support, it has four physical devices to work with:

/dev/sda  251.0 GB
/dev/sdb  251.0 GB
/dev/sdc  251.0 GB
/dev/sdd  251.0 GB

Next, partition these for use with LVM. You can do this with fdisk by specifying the "Linux LVM" partition type 8e. The finished product looks like this:

# fdisk -l /dev/sdd

Disk /dev/sdd: 251.0 GB, 251000193024 bytes
255 heads, 63 sectors/track, 30515 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

Device            Start   End      Blocks      Id  System
/dev/sdd1         1       30515    245111706   8e  Linux LVM

Notice the partition type is 8e, or "Linux LVM."

Creating a Virtual Volume

Initialize each of the disks using the pvcreate command:

# pvcreate /dev/sda /dev/sdb /dev/sdc /dev/sdd

This sets up all the partitions on these drives for use under LVM, allowing creation of volume groups. To examine available PVs, use the pvdisplay command. This system will use a single-volume group named datavg:

# vgcreate datavg /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1

Use vgdisplay to see the newly created datavg VG with the four drives stitched together. Now create the logical volumes within them:

# lvcreate --name medialv  --size 400G
# lvcreate --name backuplv --size  50G
# lvcreate --name sharelv  --size  10G

Without LVM, you might allocate all available disk space to the partitions you're creating, but with LVM, it is worthwhile to be conservative, allocating only half the available space to the current requirements. As a general rule, it's easier to grow a filesystem than to shrink it, so it's a good strategy to allocate exactly what you need today, and leave the remaining space unallocated until your needs become clearer. This method also gives you the option of creating new volumes when new needs arise (such as a separate encrypted file share for sensitive data). To examine these volumes, use the lvdisplay command.
Now you have several nicely named logical volumes at your disposal:

/dev/datavg/backuplv     (also /dev/mapper/datavg-backuplv)
/dev/datavg/medialv      (also /dev/mapper/datavg-medialv)
/dev/datavg/sharelv      (also /dev/mapper/datavg-sharelv)