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Tuesday, June 19, 2007

Adding Hard Drives to a Linux System using fdisk

In recent releases of Fedora Linux (e.g. Fedora 6 and 7), the default filesystem configuration uses Logical Volumes, which are a flexible way to manage the hard drives on a machine. But this flexibility can come at a cost if and when the drives go bad.

The old school way of setting up drives is to use the fdisk command to set up specific, fixed partitions on each drive. This requires a bit more planning and a little more execution, but it has the advantage that of a machine fails for some reason, you can still move the drives to a new machine and mount them directly.

I wanted to do that on a system recently where I had 2 x 500GB drives set up as a logical volume and then wanted to add 2 more drives to use as live backup disks for other servers. I want backup disks to be as independent as possible and so adding them to the logical volume was not a good idea. Happily you can mix logical volumes and disks with traditional partitions with no problems.

Here are the steps needed to add two drives to the Fedora 7 system. The same information is available in a load of other places. I'm including it here largely for my own benefit next time I need to do this. All steps assume you are logged in as root.

1: See what physical disks you have on your system

# /sbin/fdisk -l

You'll get a load of output - look for lines that delimit each of the physical disks, such as:
Disk /dev/sda: 500.1 GB, 500107862016 bytes

Below these you will see the partitions on each drive. Note the distinction between a disk (/dev/sda) and a partition on that disk (/dev/sda1, /dev/sda2...)

On my system the disks are SATA drives and these appear under these names /dev/sda, /dev/sdb, /dev/sdc, /dev/sdd.

/dev/sda and /dev/sdb are used in the original Logical Volume that I set up. I'm going to set /dev/sdc and /dev/sdd as two separate disks, each with a single partition on each.

2: Use fdisk to create a single partition on each disk

# /sbin/fdisk /dev/sdc
Then in response to the various prompts give these answers:
n (create a new partition)
p (make it a primary partition)
1 (call this partition #1)
hit enter to start at the first sector
hit enter again to end at the last sector (in other words make the partition fill the entire drive)
w (write the partition to the disk and quit the program)

I repeated this for disk /dev/sdd

3: Create a filesystem on each drive

The standard Linux filesystem type at the moment is ext3, although alternate filesystems can be used. Each filesystem gets a label, which is typically the same as the mount point where you will access the drives. On my system I called the two filesystem /backup0 and /backup1. You create the filesystem with the appropriate variant of the mkfs command. Note that you now refer to the specific partition (/dev/sdc1) and not the disk itself (/dev/sdc).

# /sbin/mkfs.ext3 -L /backup0 /dev/sdc1
# /sbin/mkfs.ext3 -L /backup0 /dev/sdc1

Each command will produce quite a bit of output and may take some time as it creates blocks on the actual drives.

4: Create the mount points for each filesystem

A mount point is the Linux path/directory where this filesystem can be accessed under Linux. In my case the commands were:

# mkdir /backup0
# mkdir /backup1

5: Specify these mount points in the filesystem table file
Make a copy of the existing file - just in case
# cp /etc/fstab /etc/fstab.bak

Edit the current file (/etc/fstab) and add a line for each of the new filesystem.

LABEL=/backup0 /backup0 ext3 defaults 1 2
LABEL=/backup1 /backup1 ext3 defaults 1 2

The label is an alias for the actual partition. The next field is the mount point (same name as the label in this case). The next field is the type of filesystem (ext3) and the final 3 fields are just default options that you should include.

6: Reboot the system

# /sbin/shutdown -r now

You can re-mount the filesystems with 'mount -a' but a full reboot is a safe bet.

Check that the filesystems are mounted using 'df', 'cd' to the mount points and try writing to the disks. You will want to set the directory permissions to something appropriate using chmod and chown.

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