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Raspberry Pi: Reducing file system to fit on smaller SD Card
written December 22, 2021 by TimeTraveler
Category: BlogEntry Tags: Debian; Raspberry Pi; Raspbian; resize2fs #41
For various projects, we have done some things on a Raspberry Pi. There are now multiple "flavors" of OSes for the RPi, but this example uses raspbian, a customized version of Debian for the Pi.
These are just loose notes on resizing an SD card to a smaller size for a Raspberry Pi. If moving from a smaller card to a larger card, you just need to manage the partitions - you may still want to use resize2fs, but you don't have to worry about size, like you do if shrinking the file system. There are various options, and your best bet is to review Raspberry Pi Forums, as you may find something more relevant to what your actual situation is. However, there were several commands that are interesting, and documenting the general approach and indicating how to use these commands may be useful for future reference.
As a quick reference, if only enlarging, there are just a few key steps.
- Use fdisk to enlarge the partition. Review the exact layout to ensure the correct starting sector/location. Because you need to delete the appropriate partition, then re-create it, so it must start at the same spot as the one you just deleted. Note until you write, you can still reverse back out of fdisk.
- Then run e2fsck
- Then run resize2fs
So this was a RPi 3 with a 8GB SD card, with no extra 8GB SD card anywhere to be found, but several 2GB cards were available. The files in use for the root filesystem was 1.6GB, so just enough room to make it all work.
Note these examples are working with a standalone PC, not working with the RPi itself - again, if that is all you have, the RPi Forums are a great resource.
Here is an approach that can work if you were using the exact same SD card (i.e. cloning the existing system):
NOTE: All commands done as root (superuser or user with sufficient privileges)
Create the image from the SD Card:
dd if=/dev/8gbsd of=/home/user/pi_images/8gbsd_image.img
-or could be something like-
dd if=/dev/sdb of=/home/user/pi_images/8gbsd_image.img
Restore an image to disk:
dd if=/home/user/pi_images/8gbsd_image.img of=/dev/8gbsd
-or-
dd if=/home/user/pi_images/8gbsd_image.img of=/dev/sdb
Now if you try and use a 2GB card, e.g.
dd if=/home/user/pi_images/8gbsd_image.img of=/dev/2gbsd
-or-
dd if=/home/user/pi_images/8gbsd_image.img of=/dev/sdb
You will find there will be some warnings during the dd and that the 2GB SD card won't boot (kernel panic or file system issue)
So what to do?
You need some common / readily available utilities - GPartEd (GNOME Partition Editor), fdisk, e2fsck, resize2fs, e4defrag
If not installed/available, these are debian packages for reference:
e2fsprogs (e4defrag resize2fs e2fsck), fdisk package (fdisk), gparted package (gparted)
You need a scratch disk or USB flash drive or other media to do your resize work (theoretically you could use the 8GB card, but this was a working system, and there was no intention of modifying the original working system on the 8GB SD Card - so image it, and then put the working 8GB SD card aside)
For this example, a 16GB USB flash drive was used.
Partition as Linux (fdisk), and then format ext4 (mkfs.ext4)
Make sure it is not mounted, and then duplicate ext4 partition from 8GB card (both SD card and USB inserted, e.g. /dev/sdb and /dev/sdc)
dd if=/dev/sdb2 of=/dev/sdc1 (grab partition 2 from SD card and drop onto the USB drive)
Test USB drive
mount /dev/sdc1 /mnt
ls /mnt
You should see duplicated root file system on USB drive
Now prep the drive - you can delete any files you know aren't needed, then defragment the drive to optimize the layout of the file system, e.g.
/sbin/e4defrag /dev/sdc1
Then do file system check / prep for resize
/sbin/e2fsck -f /dev/sdc1
Now unmount the partition for resize step
umount /dev/sdc1
Finally, use the resize2fs utility to resize the file system
You can use this to see details on minimum size needed:
/sbin/resize2fs -P /dev/sdc1
The -M option minimizes the file system at optimum size:
/sbin/resize2fs -M /dev/sdc1
If this isn't small enough, you need to make sure you can still fit the files onto the 2GB card. If the usage is 1.6 GB used, you can try. If it is 2.6GB, there is no way the 2GB card will work. You can go back to the Test step, mount the USB, and try and delete files, then repeat the steps.
You can force the size with the -f option - note that if the existing files are too large to fit in your specified size, you can run into corrupted files, or it simply won't work.
/sbin/resize2fs -f /dev/sdc1 1820M
So now you've got a reduced ext4 partition that can fit on the 2GB card, but the 2GB card needs to be structured so it will boot on the RPi.
Start with gparted, and document what partitions are on the 8 GB card, note start point, size, and number of sectors per partition.
Now drop the 8GB image onto the 2GB - this will ensure the beginning part of the drive will match the layout of the boot partition.
dd if=/home/user/pi_images/8gbsd_image.img of=/dev/sdb
You can use gparted or fdisk, but you want to reduce the second partition so it will fit on the 2GB SD card. With fdisk, you'd delete partition 2, make a new one, and then select/verify the correct start sector and fit the maximum size for the new second partition. When asked about ext4 signature, you do not want to delete.
Again, you can use gparted to view & compare the 8GB vs. the 2GB - the beginning should be the same, and the second ext4 partition should fit on the card.
Now, you can put the reduced root partition from your USB drive:
dd if=/dev/sdc1 of=/dev/sdb2 (grab the USB drive and drop onto partition 2 from SD card)
Now, if the sizes are all good, and no errors or warnings, you can remove the USB drive, and try and mount partition 2/ext4 root file system:
mount /dev/sdb2 /mnt
ls /mnt
If all looks good, unmount the drive:
umount /dev/sdb2
Now put the SD card into your Raspberry Pi and Boot Away!
Here are a few other notes regarding layout on SD card (for reference):
Clone MBR + BootSector to a new drive:
dd if=/dev/sdb of=/dev/sdc bs=512 count=2
To restore the MBR from a backup image you want to copy only the first 446 bytes:
dd if=mbrbackup.img of=/dev/sdc bs=446 count=1
Clone only MBR from source disk to a new drive:
dd if=/dev/sdb of=/dev/sdc bs=446 count=1
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