Repartitioning a CompactFlash Disk for Linux

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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.

  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.
  4. Determine the device node for the CF disk. There are several methods that can be used to accomplish this:
    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/sdc: 2048 MB, 2048901120 bytes
224 heads, 56 sectors/track, 319 cylinders
Units = cylinders of 12544 * 512 = 6422528 bytes
Disk identifier: 0x6a5f7029

   Device Boot      Start         End      Blocks   Id  System
/dev/sdc1   *           1        1008     6322175+  83  Linux

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
      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.
  5. 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. First, mount 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 will prevent a full archive from being obtained. 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
  6. 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
  7. Hel Help
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