Fedora Linux

6.1.3.2. ...improving performance?

The LVM system has striping capability, which spreads data over multiple PVs. Data can be read from multiple PVs simultaneously, increasing throughput in some cases.

To enable striping, use the -i (stripe-count) and -I (stripe-size) arguments to the lvcreate command:

# lvcreate main -i 3 -I 8 --name mysql --size 20G

The stripe count must be equal to or less than the number of PVs in the VG, and the stripe size (which is in kilobytes) must be a power of 2 between 4 and 512.

You can also select striping in the LV Properties area of the Create New Logical Volume dialog ( Figure 6-4 ).

6.1.3.3. ...LVM mirroring?

To protect data integrity, recent versions of LVM provide a mirroring capability, which stores two copies of each physical extent on two different disks. However, this is noted as a technology preview capability in Fedora Core 6, meaning that it's at a beta-test stage.

An alternative approach that is stable, proven, and provides a wider range of configuration options is to layer LVM on top of the md RAID system (discussed in Lab 6.2, 'Managing RAID ').

6.1.3.4. ...using LVM with RAID?

LVM can be layered on top of the Linux md RAID driver, which combines the flexibility of LVM with striping, mirroring, and advanced error-correction capabilities. See Lab 6.2, 'Managing RAID ,' for details on how this is configured.

6.1.3.5. ...using a raw, unpartitioned disk as a PV?

Although you can use a raw disk as a PV, it's not recommended. The graphical administration tools don't support it, and the amount of space lost to a partition table is minimal (about 1 KB).

6.1.3.6. ...a failing disk drive?

If you suspect that a disk drive is failing, and you want to save the data that is on that drive, you can add a replacement PV to your volume group, migrate the data off the failing (or slow or undersized) disk onto the new PV, and then remove the original disk from the volume group.

To migrate data off a specific PV, use the pvmove command:

# pvmove /dev/hda3

6.1.3.7. ...creating a flexible disk layout?

LVM is all about flexibilitybut for absolute maximum flexibility, divide your disk into multiple partitions and then add each partition to your volume group as a separate PV.

For example, if you have a 100 GB disk drive, you can divide the disk into five 20 GB partitions and use those as physical volumes in one volume group.

The advantage to this approach is that you can free up one or two of those PVs for use with another operating system at a later date. You can also easily switch to a RAID array by adding one (or more) disks, as long as 20 percent of your VG is free, with the following steps:

1. Migrate data off one of the PVs.

2. Remove that PV from the VG.

3. Remake that PV as a RAID device.

4. Add the new RAID PV back into the VG.

5. Repeat the process for the remaining PVs.

You can use this same process to change RAID levels (for example, switching from RAID-1 (mirroring) to RAID-5 (rotating ECC) when going from two disks to three or more disks).

6.1.4. Where Can I Learn More?

? The manpages for lvm , vgcreate , vgremove , vgextend , vgreduce , vgdisplay , vgs , vgscan , vgchange , pvcreate , pvremove, pvmove , pvdisplay , pvs , lvcreate , lvremove , lvextend , lvreduce , lvresize , lvdisplay , lvs

? The LVM2 Resource page: http://sourceware.org/lvm2/

? A Red Hat article on LVM: http://www.redhat.com/magazine/009jul05/departments/red_hat_speaks/

6.2. Managing RAID

Redundant Arrays of Inexpensive Disks (RAID) is a technology for boosting storage performance and reducing the risk of data loss due to disk error. It works by storing data on multiple disk drives and is well supported by Fedora. It's a good idea to configure RAID on any system used for serious work.