As shown in Part 1 of this article, extracting Linux from a pre-made .img file can result in unused space at the end of the uSD card. Depending on the size of the uSD card, this could leave a significant amount of space left unused. This article shows how to repartition the uSD card to utilize all of the space on the uSD card. This can be done on an unmounted filesystem or even on a live filesystem. In the example below, the live filesystem of a BeagelBone Black (running the Debian Wheezy image) will be resized to take advantage of the full 4GB of space available on the uSD card used in this example.
Step 1: Start fdisk
To get started, list the volumes available on a BeagleBone Black that is booted from a uSD card:
root@debian-armhf:/# ls -l /dev/mmcblk*
brw-rw---T 1 root floppy 179, 0 Jan 1 2000 /dev/mmcblk0
brw-rw---T 1 root floppy 179, 1 Jan 1 2000 /dev/mmcblk0p1
brw-rw---T 1 root floppy 179, 2 Jan 1 2000 /dev/mmcblk0p2
brw-rw---T 1 root floppy 179, 8 Jan 1 2000 /dev/mmcblk1
brw-rw---T 1 root floppy 179, 16 Jan 1 2000 /dev/mmcblk1boot0
brw-rw---T 1 root floppy 179, 24 Jan 1 2000 /dev/mmcblk1boot1
brw-rw---T 1 root floppy 179, 9 Jan 1 2000 /dev/mmcblk1p1
brw-rw---T 1 root floppy 179, 10 Jan 1 2000 /dev/mmcblk1p2
The listing above shows an external uSD card is currently booted as indicated by the /dev/mmcblk0 entries and the lack of /dev/mmcblk0boot entries. When booted from the internal eMMC, the device with the /dev/mmcblk1boot entries would instead be at the device zero location as shown here:
root@debian-armhf:/# ls -l /dev/mmcblk*
brw-rw---T 1 root floppy 179, 0 Jan 1 2000 /dev/mmcblk0
brw-rw---T 1 root floppy 179, 1 Jan 1 2000 /dev/mmcblk0p1
brw-rw---T 1 root floppy 179, 16 Jan 1 2000 /dev/mmcblk0boot0
brw-rw---T 1 root floppy 179, 24 Jan 1 2000 /dev/mmcblk0boot1
brw-rw---T 1 root floppy 179, 2 Jan 1 2000 /dev/mmcblk0p2
brw-rw---T 1 root floppy 179, 8 Jan 1 2000 /dev/mmcblk1
brw-rw---T 1 root floppy 179, 9 Jan 1 2000 /dev/mmcblk1p1
brw-rw---T 1 root floppy 179, 10 Jan 1 2000 /dev/mmcblk1p2
To get started, run fdisk /dev/mmcblk0 to examine the partitioning of the external uSD card that is currently booted:
root@debian-armhf:/# fdisk /dev/mmcblk0
Command (m for help): p
Disk /dev/mmcblk0: 3947 MB, 3947888640 bytes
4 heads, 16 sectors/track, 120480 cylinders, total 7710720 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x80000000
Device Boot Start End Blocks Id System
/dev/mmcblk0p1 * 2048 4095 1024 1 FAT12
/dev/mmcblk0p2 4096 3751935 1873920 83 Linux
Command (m for help):
Take a moment to examine what fdisk is reporting. The first line indicates that the uSD card is 3947888640 bytes in size. The 4th line reports the sector size to be 512 bytes. Some quick math shows that 3947888640 bytes / 512 bytes per sector = 7710720 sectors as is also reported on the second line. The default “block” size is 1024 bytes, but it does not actually report this unit anywhere. The Start and End columns are reporting 512 byte sectors, and the Blocks column is reporting 1024 byte blocks. The way this .img is partitioned makes the relationship easy to see in this case, but it would not be immediately apparent if this were a large hard disk and the numbers were not as magic. The starting block of the partition table at position 2048 reveals that blocks 0-2047 are not being used for user data. This amounts to a full megabyte (2048 * 512 bytes per block = 1048576 bytes), and this is the default of fdisk. The first partition is 2048 sectors in size, or 1024K, which confirms the block size being 1K.
To calculate the total size of the space used on the disk, there are 2048 sectors at the head reserved for partitioning, 2048 sectors in mmcblk0p1, and 3,751,936 sectors in mmcblk0p2 for a total of 3,756,032 sectors. With each sector using 512 bytes, this totals 1,923,088,384 bytes. The choice of 1,923,088,384 bytes was used to make this image the exact size of the BeagleBone’s available eMMC space (note that 1,923,088,384 bytes / 1024^2 = 1834 MB). It is also a good size because not all external 2 GB uSD cards are exactly 2048 * 1024^2 in size. Having it a bit under 2 GB makes it a sure fit.
Repartitioning the disk is rather easy since fdisk will prompt with smart default choices. The steps below will begin by deleting partition 2, then recreate it as a larger size, and finally write the new table to the disk only at the end. Notice the emphasis on the last part: No changes are actually committed to disk along the way — only by pressing ‘w’ at the end will cause changes to be written. If you make a mistake or panic, just hit ‘q’ to quit and no changes will have been made.
Step 2: Delete Partition 2
Press ‘d’ for delete and ‘2’ for partition 2.
Command (m for help): p
Disk /dev/mmcblk0: 3947 MB, 3947888640 bytes
4 heads, 16 sectors/track, 120480 cylinders, total 7710720 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x80000000
Device Boot Start End Blocks Id System
/dev/mmcblk0p1 * 2048 4095 1024 1 FAT12
/dev/mmcblk0p2 4096 3751935 1873920 83 Linux
Command (m for help): d
Partition number (1-4): 2
Command (m for help): p
Disk /dev/mmcblk0: 3947 MB, 3947888640 bytes
4 heads, 16 sectors/track, 120480 cylinders, total 7710720 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x80000000
Device Boot Start End Blocks Id System
/dev/mmcblk0p1 * 2048 4095 1024 1 FAT12
Command (m for help):
Step 3: Recreate Partition 2
Press ‘n’ for new, ‘p’ for primary, and ‘2’ for partition 2. Specify start and end sectors for the new partition — just select the default values by pressing enter. In fact, outside of the the first ‘n’ they were all default choices and pressing enter alone to confirm the choice is all that is needed.
Command (m for help): n
Partition type:
p primary (1 primary, 0 extended, 3 free)
e extended
Select (default p): p
Partition number (1-4, default 2): 2
First sector (4096-7710719, default 4096):
Using default value 4096
Last sector, +sectors or +size{K,M,G} (4096-7710719, default 7710719):
Using default value 7710719
Command (m for help): p
Disk /dev/mmcblk0: 3947 MB, 3947888640 bytes
4 heads, 16 sectors/track, 120480 cylinders, total 7710720 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x80000000
Device Boot Start End Blocks Id System
/dev/mmcblk0p1 * 2048 4095 1024 1 FAT12
/dev/mmcblk0p2 4096 7710719 3853312 83 Linux
Command (m for help):
That is it! Select ‘w’ to commit the changes to the uSD card. Notice that the partition table in this example was “busy” so a reboot was needed to cause the changes to be reflected. Even if it were not busy, it seems like it could be a good idea to reboot at this point if you want to be extra safe.
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
WARNING: Re-reading the partition table failed with error 16: Device or resource busy.
The kernel still uses the old table. The new table will be used at
the next reboot or after you run partprobe(8) or kpartx(8)
Syncing disks.
root@debian-armhf:/# reboot
Step 4: Expand the Filesystem
This is the last step. Now that the second partition is larger, expand the filesystem to match the larger partition using resize2fs.
root@debian-armhf:/# df
Filesystem 1K-blocks Used Available Use% Mounted on
rootfs 1811704 740184 977824 44% /
/dev/root 1811704 740184 977824 44% /
devtmpfs 253920 0 253920 0% /dev
tmpfs 50816 216 50600 1% /run
tmpfs 5120 0 5120 0% /run/lock
tmpfs 101620 0 101620 0% /run/shm
/dev/mmcblk0p1 1004 474 530 48% /boot/uboot
root@debian-armhf:/#
root@debian-armhf:/# resize2fs /dev/mmcblk0p2
resize2fs 1.42.5 (29-Jul-2012)
Filesystem at /dev/mmcblk0p2 is mounted on /; on-line resizing required
old_desc_blocks = 1, new_desc_blocks = 1
The filesystem on /dev/mmcblk0p2 is now 963328 blocks long.
root@debian-armhf:/# df
Filesystem 1K-blocks Used Available Use% Mounted on
rootfs 3761680 741096 2851404 21% /
/dev/root 3761680 741096 2851404 21% /
devtmpfs 253920 0 253920 0% /dev
tmpfs 50816 216 50600 1% /run
tmpfs 5120 0 5120 0% /run/lock
tmpfs 101620 0 101620 0% /run/shm
/dev/mmcblk0p1 1004 474 530 48% /boot/uboot
root@debian-armhf:/#
Summary
All of the space on the uSD card is now available for use. To recap:
# fdisk /dev/mmcblk0
d
2
n
p
2
4096
w
# reboot
# resize2fs /dev/mmcblk0p2