Repartitioning a CompactFlash Disk for Linux

Many embedded systems require writing to persistent storage during routine operation. While the journaling filesystems used on EMAC OE systems are tolerant of power failures during a write to the filesystem, it is still advantages to leave the root filesystem mounted read-only as often as possible to prevent any possibility of data loss or corruption on the root filesystem. One method to accomplish this is using a separate disk (such as an SD card) or secondary partitions on the primary storage device for any application-specific data that needs to be written to disk. This article describes the process of creating and utilizing an additional partition for data storage.

CompactFlash Partitioning

The majority of EMAC's x86-based systems utilize CompactFlash as the primary storage device. By default, EMAC OE is provided with a single ext3 partition on the CompactFlash holding the root filesystem. The most efficient method for repartitioning this type of device is using a CompactFlash card reader on a Linux PC. While command-line tools can be utilized, the application gparted is a graphical application that works very well for resizing existing partitions. Follow the procedure below to resize the existing partition and create a secondary partition on a CompactFlash.

Setup

1. Power down the target board and carefully remove the CF disk.
2. Insert the CF disk into the CF reader connected to a Linux PC.
3. The system will most likely bring up a window asking what to do with the device once it is detected. If this occurs, select "Do Nothing" or a similar option to indicate that the device should not be mounted automatically.

Determine which Device to Use

Next you must determine the device node corresponding to the CF disk. There are several methods that can be used to accomplish this as described in the procedure below:

1. dmesg can provide information about the disk that was reported by the kernel when it was detected as shown below.

   developer@ldc:~$ dmesg | tail
   [7414822.178026] sd 6:0:0:1: [sdc] 4001760 512-byte logical blocks: (2.04 GB/1.90 GiB)
   [7414822.179094] sd 6:0:0:1: [sdc] Assuming drive cache: write through
   [7414822.180711] sd 6:0:0:1: [sdc] Assuming drive cache: write through
   [7414822.180716]  sdc: sdc1
   

   In this example, sdc is the device corresponding to the CF disk. Also note that the size reported matches the expected size for the CF disk (2 GB in this case). The output also shows that a single partition was detected on the device: sdc1.
2. The fdisk command can also be used to list the available devices using the -l options. Note that you may or may not have permissions to run the fdisk command as a normal user, it may be necessary to run the command as root using sudo or su. An example fdisk -l output is shown below.

   developer@ldc:~$ sudo /sbin/fdisk -l
   Disk /dev/sda: 500.1 GB, 500107862016 bytes
   255 heads, 63 sectors/track, 60801 cylinders
   Units = cylinders of 16065 * 512 = 8225280 bytes
   Disk identifier: 0x00000000
   
      Device Boot      Start         End      Blocks   Id  System
   /dev/sda1   *           1       60302   484375783+  83  Linux
   /dev/sda2           60303       60801     4008217+   5  Extended
   /dev/sda5           60303       60801     4008186   82  Linux swap / Solaris
   
   Disk /dev/sdf: 2000.3 GB, 2000398934016 bytes
   255 heads, 63 sectors/track, 243201 cylinders
   Units = cylinders of 16065 * 512 = 8225280 bytes
   Disk identifier: 0x0b45dffc
   
      Device Boot      Start         End      Blocks   Id  System
   /dev/sdf1               1      243201  1953512001   83  Linux
   
   Disk /dev/sdc: 2048 MB, 2048901120 bytes
   64 heads, 62 sectors/track, 1008 cylinders
   Units = cylinders of 3968 * 512 = 2031616 bytes
   Disk identifier: 0x6a5f7029
   
      Device Boot      Start         End      Blocks   Id  System
   /dev/sdc1   *           1        1008     1999871+  83  Linux
   

   Three devices are listed in this output: /dev/sda, /dev/sdc, and /dev/sdf. Looking at the disk sizes, /dev/sdc matches the 2 GB size of the CF card (2048 MB as listed in the output above).
3. After checking the dmesg and fdisk output, inspect the mount locations on the system to verify that the correct disk is being used and that it has not been utilized by the system. In the example below, the grep command is used to search the output of the mount command for the string "/dev/sd".

   developer@ldc:~$ mount | grep /dev/sd
   travis@cheeto:~$ mount | grep /dev/sd
   /dev/sda1 on / type ext3 (rw,errors=remount-ro)
   /dev/sdf1 on /media/backup type ext3 (rw)
   

   This output illustrates that /dev/sda1 is the root filesystem, /dev/sdf1 is an auxiliary disk mounted on /media/backup, and /dev/sdc1 is not mounted. Given the results from these three commands, /dev/sdc is the CF disk. If /dev/sdc1 is mounted, it should be unmounted before continuing.
   Do not continue if you are not certain which device node corresponds to the CF disk.

Backup the Existing Filesystem

After determining the CF disk on the system, make a backup of the CF disk to ensure that it can be restored if something goes wrong during the repartitioning process. The steps below assume that /dev/sdc is the CF disk as determined in the example above. Replace the /dev/sdc occurrences with the appropriate device for your system.

1. Begin by mounting the CF disk. It is important to do this manually rather than having the system auto-mount the device to prevent it from being mounted with options that could make it difficult to obtain a complete archive. The listing below demonstrates how to mount the CF disk to a temporary directory:

   developer@ldc:~$ sudo mkdir /tmp/cf
   developer@ldc:~$ sudo mount /dev/sdc1 /tmp/cf
   

   After mounting, inspect the files at the mount point and verify that the filesystem is there as expected:

   developer@ldc:~$ ls /tmp/cf
   bin  boot  dev  etc  home  lib  lost+found  media  mnt  proc  root  sbin  sys  tmp  usr  var
   

2. Use tar to create a gzipped tar archive of the entire flash. The following commands will create a backup archive named /home/developer/rootfs-backup/rootfs_20130404.tar.gz:

   developer@ldc:~$ cd /tmp/cf
   developer@ldc:~$ sudo tar czvf ~/rootfs-backup/rootfs_20130404.tar.gz *
   

3. Unmount the CF disk once the backup is complete:

   developer@ldc:~$ cd
   developer@ldc:~$ sudo umount /dev/sdc1
   

Repartition the CF Disk using Gparted

At this point you will need to decide on a partition scheme to use. In this example, the primary partition and ext3 filesystem will be resized to 200 MB and a new primary partition will be created. Depending on your application requirements, you could create multiple primary partitions, or an extended partition with multiple logical partitions.

Follow the steps below to repartition the CF disk.

1. Start the gparted utility, passing the CF disk device node as an argument as shown below:

   developer@ldc:~$ sudo gparted /dev/sdc
   

   This should bring up a window similar to the one shown in Figure 1 below.
   

   Figure 1: Gparted Initial View
2. Click on the partition to select it and press the Resize/Move button. Enter 200 in the New Size (MiB) setting in the new window that appears as shown in Figure 2 below. Press Resize/Move to queue this operation and return to the main window.
3. You should now see a the primary partition followed by a large unallocated section. Although it is acceptable to continue making all changes before applying the changes, it may be better to apply changes in steps. Press the "Apply" button to execute the pending operations required to resize the root partition and filesystem. A window will appear with the operations and their status. You can view details for each operation as it is executed. See Figure 3 below for an example. Press "Close" to return to the main window.
4. To create a new partition in the unallocated portion of the disk, click on the unallocated section to select it, and press the "New" button. A new window will appear to enter the details for the partition. After entering the settings below (also shown in Figure 4), press "Add" to queue the required operations.
   1. Create as: Primary partition
   2. Filesystem: ext3
   3. Use the maximum allowed size for the New Size setting
5. At the main window the device layout should reflect the new partition as shown below in figure 5. Press "Apply" to commit the changes to the disk.
6. After all operations finish, verify the device layout and partition table as shown in the main window of gparted . In Figure 6 you can see that /dev/sdc1 is the resized root filesystem and that /dev/sdc2 is the new data partition.
7. Close gparted and remove the CF disk from the reader.

Configure the Target Board

After repartitioning the CF disk, it is necessary to configure the target board to tell it where and how to mount the new partition. In this example, the new partition will be mounted on /mnt/data but any mount point may be used.

1. Insert the CF disk into the target board and apply power to the system.
2. Log in as the root user after the system has booted.