How to set /dev/root filesystem size to the partition size - yocto

Question

# 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?

Answer 1

I can see two possible solutions to your issue.
The first one is by telling Yocto to generate an image with a sepcific 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 a SDcard, 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.

Answer 2

As PierreOlivier proposed I used resize2fs program.
Because I am using Yocto to build the custom image I use pkg_postinst_ontarget which runs only once at the first boot on the target machine.
In one of my recipe I put
pkg_postinst_ontarget_${PN}() {
#!/bin/bash
resize2fs /dev/mmcblk1p4
}
This resizes the selected partition on the first boot.
Thank you PierreOlivier