with emmc unmounted

sudo e2fsck -f /dev/mmcblk1p1 # check filesystem
sudo resize2fs -M /dev/mmcblk1p1 # shrink filesystem to minimum size
sudo fdisk /dev/mmcblk1 # shink partition
sudo resize2fs /dev/mmcblk1p1 # Expand filesystem to fill the new (smaller) partition
sudo partprobe /dev/mmcblk1 # refresh kernel partition table

p – print table (to get start sector of p1)
d – delete partition
n – create new primary partition #1
  Use same start sector
  Choose a smaller size that is slightly larger than the new filesystem
w – write changes

• Start sector was: 8192
• New end sector: something corresponding to ~5.2GB:
  • 1 GiB = 1,073,741,824 bytes
  • 5.2 × 1,073,741,824 = 5,583,457,484.8 bytes
  • 5,583,457,485 / 512 ≈ 10,910,860 sectors
  • end = 8192 + 10,910,860 - 1 = 10,919,051

so that:

• temporary files created by programs go to RAM
• keeps logs/temp writes from wearing out eMMC
• mode=1777: world-writable sticky bit (normal for /tmp)
• limit size of files/logs

inside /etc/fstab

tmpfs /tmp      tmpfs defaults,noatime,mode=1777,size=128M 0 0
tmpfs /var/tmp  tmpfs defaults,noatime,mode=1777,size=64M  0 0
tmpfs /var/log  tmpfs defaults,noatime,mode=0755,size=32M  0 0

• you'll have no logs for boot failures
• inside /etc/systemd/journald.conf

Storage=volatile
RuntimeMaxUse=10M
RuntimeKeepFree=2M
SystemMaxUse=0   # ignored in volatile mode

• in /etc/fstab: /dev/mmcblk1p1 / ext4 noatime,data=writeback,barrier=1,commit=60 0 1
  • noatime: don't update access time every time you read a file
  • data=writeback: lighter journaling (metadata-only)
  • commit=60: group writes once per minute instead of 5 seconds
  • barrier=1: maintain safety
  • data=writeback + sudden power loss = some recently changed files may become garbage inside (but no FS corruption)