On Non-Default SD Image File Sizing

SD image files are sized to strike a balance between the storage required for the OS itself, the amount of free storage desired (or required) when the image "goes live", and the bandwidth required to move image files between workstations.

The size of the image file itself can be adjusted when the image is constructed. Or, the size of the image's filesystem can be expanded after the image file is written to an SD card.

Procedure - Specifying an Image Size at Construction

The FSSIZZEMB envar defines the image file's final size. You can override the default at the mkos command line:

user@workstation:$ FSSIZEMB=4096 mkos-akna

Resizing a Deployed Image

Advanced users will prefer the following alternative procedure in some cases:

Build SD image files using the smallest FSSIZEMB possible; then,
Resize the image after writing the SD card.

Using the smallest-possible FSSIZEMB accelerates both image-generation and distribution activities. Resizing the image's filesystem after writing the image to an SD card allows the runtime free storage to be adjusted depending upon the size of the SD card in use.

The resizing step requires the fdisk and resize2fs tools, and can be performed at the user's workstation or on a live filesystem (a reboot is usually required in the latter case).

(Note: sfdisk can be used in place of fdisk, but the details are beyond the scope of this document.)

Procedure - Resizing an Installed Filesystem

First, resize the partition containing the image's filesystem.

Assuming your SD card is attached at /dev/sdb, delete and then re-create the filesystem partition beginning at the same starting sector. The end sector must be equal to or greater than the original location:

user@workstation:~$ sudo fdisk /dev/sdb
Welcome to fdisk (util-linux 2.33.1).
Command (m for help): p
Disk /dev/sdb: 29.7 GiB, 31914983424 bytes, 62333952 sectors
Device Boot Start End Sectors Size Id Type
/dev/sdb1 * 4096 4194303 4190208 2G 83 Linux

Command (m for help): d
Selected partition 1
Partition 1 has been deleted.

Command (m for help): n
Partition type
Select (default p): p
Partition number (1-4, default 1): 1
First sector (2048-62333951, default 2048): 4096
Last sector, +/-sectors or +/-size{K,M,G,T,P} (4096-62333951, default 62333951): 62333951
Created a new partition 1 of type 'Linux' and of size 29.7 GiB.

Partition #1 contains a ext4 signature.
Do you want to remove the signature? [Y]es/[N]o: n

Command (m for help): w
The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.

Second, resize the filesystem so that it fills the whole partition:

user@workstation:~$ sudo resize2fs /dev/sdb1
resize2fs 1.44.5 (15-Dec-2018)
Resizing the filesystem on /dev/sdb1 to 7791232 (4k) blocks.
The filesystem on /dev/sdb1 is now 7791232 (4k) blocks long.

You may now use the SD card as usual.