# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/root 4.3G 1.9G 2.2G 47% /
devtmpfs 980M 0 980M 0% /dev
tmpfs 981M 0 981M 0% /dev/shm
tmpfs 981M 33M 948M 4% /run
tmpfs 981M 0 981M 0% /sys/fs/cgroup
tmpfs 981M 0 981M 0% /tmp
tmpfs 981M 16K 981M 1% /var/volatile
# fdisk -l
Disk /dev/mmcblk1: 7.3 GiB, 7818182656 bytes, 15269888 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
Disklabel type: gpt
Disk identifier:
Device Start End Sectors Size Type
/dev/mmcblk1p1 16384 24575 8192 4M unknown
/dev/mmcblk1p2 24576 32767 8192 4M unknown
/dev/mmcblk1p3 32768 69859 37092 18.1M unknown
/dev/mmcblk1p4 81920 15269854 15187935 7.2G unknown
As far as I know, the /dev/root filesystem size is the size of the content that is being copied to the /dev/root.
My goal is to have /dev/root size the same as /dev/mmcblk1p4 which is 7.2G.
How can I instruct Yocto give the /dev/root filesystem the size of the partition it is mounted to?
I can see two possible solutions to your issue.
The first one is by telling Yocto to generate an image with a specific IMAGE_ROOTFS_SIZE value. As stated in the Yocto Mega-Manual, the size is specified in Kbytes. Modify your machine.conf or local.conf to add this parameter.
In your case, the value seems to be:
15269854 - 81920 = 15187934 sectors
sectors are 512 Bytes on your system (see verification below)
15187935 * 512 = 7776222208 Bytes
7776222208 / 1024 = 7593967 KBytes
To verify the sector size of 512B:
7593967 / (1024 * 1024) = 7.242 GB
With 512 Bytes blocksize, the partition size is 7.2GB, as stated by fdisk
I think it is a good idea to reduce it a little bit, a value like 7230000 Kilobytes (~7.23 GB) can be a good candidate.
The other method is to use resize2fs program if your partition type is ext2/3/4. This program can be executed on mounted or unmounted devices. If you are using an SD card, the simplest method will be to insert it into your computer, to unmount it, and run resize2fs /dev/<mysdcarddevice>. You can also execute it directly from your embedded board. In this case you will need to add the package e2fsprogs-resize2fs on the board with IMAGE_INSTALL += "e2fsprogs-resize2fs", then run resize2fs /dev/mmcblk1p4.