Fedora Linux

None to ext3, and select the checkboxes for Mount and 'Mount when rebooted.' In the 'Mount point' field, type the name of the directory where you wish the new filesystem to appear.

For example, to create a 10 GB partition for music and video files, you could enter an LV name of multimedia , set the size to 10 GB, and create an ext3 filesystem with a mount point of /media .

Click OK. The LV and filesystem will be created and mounted, and you can start using the filesystem immediately.

6.1.1.2.2. Creating a snapshot

LVM has the ability to create a snapshot of an LV. The snapshot is an exact copy of the LV as it stood when the snapshot was created, but this is an illusion because the snapshot really stores only a copy of data that was changed since the snapshot was created. You can change the data in the origin LV without affecting the snapshot, and change the data in the snapshot without affecting the original LV.

Snapshots enable you to make a self-consistent backup of a filesystem to media such as tape. If you don't use snapshots and you back up an active filesystem containing a database to tape, the database tables would get copied at different times; if the database contained e-commerce data, perhaps the customer table would get copied before the order table. If an order was received from a new customer while the backup was in progress, it is possible that the order table on the tape will include the order but the customer table may not include the new customer. This could lead to severe problems when trying to use the data at a later time. On the other hand, if you take a snapshot and then back that up, the various files will all be in the same state on tape.

In addition, snapshots are useful for self-administered document recovery: if you take a snapshot of your users' files each night and make that snapshot available to them, they can recover from their own mistakes if they mess up a spreadsheet or delete an important document. For example, if you take a snapshot of /home and make it available as /yesterday/home , the deleted document /home/jamie/budget.ods can be recovered as /yesterday/home/jamie/budget.ods .

Snapshots are also used to test software or procedures without affecting live data. For example, if you take a snapshot of the logical volume containing the /home filesystem, and then unmount the original filesystem and mount the snapshot in its place, you can experiment with procedures that change the contents of home directories. To undo the results of your experiments, simply unmount the snapshot, remount the original directory, and then destroy the snapshot.

To create a snapshot of a LV using the graphical tool, select the LV in the left pane, and then click on the Create Snapshot button at the bottom of the middle pane. You will see the dialog box shown in Figure 6-5 .

Figure 6-5. Creating a snapshot

This dialog looks a lot like the dialog used to create a logical volume ( Figure 6-4 ), and it shouldbecause a snapshot is a special type of LV. Enter a name for the snapshot; I recommend the name of the origin LV, with - snap added to the end. For example, a snapshot of the multimedia LV would be called multimedia-snap .

Next, set the size of the snapshot. The snapshot will appear to be the same size as the origin LV; the size setting here is used to reserve disk space to track the differences between the origin LV and the snapshot. Therefore, if you have a 100 GB LV and the data in that LV changes slowly, a 1 GB snapshot might be reasonable; but if the data in that LV changes rapidly, you will need a much larger snapshot size.

Select the Mount and 'Mount when rebooted' checkboxes, and then enter the 'Mount point' that you wish to use (such as /backup/media ).

You can view the amount of storage used by the snapshot by selecting the snapshot LV in the left pane, then looking at the snapshot usage in the right pane. The usage is reported as a percentage of the total snapshot size and increases as data is changed in the origin or snapshot volumes. If it approaches 100 percent, you can increase the size of the snapshot LV in the same way that you would resize a regular LV.

6.1.1.2.3. Removing a logical volume or a snapshot

To permanently remove a logical volume, select it in the left pane, and then click the Remove Logical Volume button at the bottom of the middle pane. A dialog box will appear, asking you to confirm your choice; when you click Yes, the logical volume will be gone forever.

6.1.1.2.4. Adding a partition

You can add a partition to a volume group at any time.

The first step is to make the partition a physical volume. Select the disk partition you wish to use under Uninitialized Entities in the left pane, and then click the Initialize Entity button at the bottom of the center pane. A dialog box will warn you of possible data loss; double-check the partition information, and then click Yes if you are certain that you will not lose any critical data.  

Be extremely careful with this option because it will delete all of the data on an entire disk partition. If you select the wrong partition on a dual-boot system, you could wipe out all of the data used by the other operating system (such as Windows).

If the Initialize Entity button is deactivated (grayed-out and unclickable), look in the right pane for the reason that the partition is 'Not initializable.' The most common reason given is Foreign boot partition , which means that the partition is marked as bootable in the drive's partition table. To correct this, use fdisk on the disk containing the partition; for example, run fdisk on the disk /dev/sdb to edit the settings for the partition /dev/sdb1 :

# fdisk /dev/sdb
fdisk accepts single-letter commands. Enter p to print the partition table:
Command (m for help): p
Disk /dev/sdb: 8 MB, 8192000 bytes
4 heads, 16 sectors/track, 250 cylinders
Units = cylinders of 64 * 512 = 32768 bytes
Device   Boot Start End Blocks Id System
/dev/sdb1 *       1 250 7987+    1 FAT12

There is only one partition on this particular disk, and it is bootable (note the * in the Boot column). Use the a (activate) command to toggle the boot flag:

Command (m for help): a
Partition number (1-4):
 1