Can I mount /data/data/(app_name)/ on external sd card?

Question

Can I mount /data/data/com.termux/ folder on external sd card? My phone is rooted Android 10 and running low of internal storage.

As I read from How to bind mount a folder inside /sdcard with correct permissions? mounting to /sdcard requires permission to mount because /sdcard is emulated from /data/media/0 and /data/data/ is not.

And from Can the Termux environment be put on an external SD card? it can mount /data/data/com.termux/ using ln -s because ext4 support links while fat doesn't.

I don't know how /data/data/ directory works but is it possible to mount /data/data/com.termux/files/home/ directory to external sd card without formatting it as adoptable storage or making ext4 partition?

Answer 1

• create a file in MicroSD Card. make sure the file is not too large for fat32 file size limitation

  busybox dd if=/dev/zero seek=4294967295 bs=1 count=0 of=/mnt/media_rw/6DE8-1BC6/loop.img
  

• create a file system (confirm with y)

  /system/bin/mke2fs -t ext4 /mnt/media_rw/6DE8-1BC6/loop.img
  

• loop mount the file to existing directory

  mkdir -p /data/data/com.termux/files/home/ubuntu20-fs
  busybox mount /mnt/media_rw/6DE8-1BC6/loop.img /data/data/com.termux/files/home/ubuntu20-fs
  

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UPDATE:

that works only for busybox with largefile support. according to user Jtnqr and user Hmm you may still facing some trouble. the following is exactly the same example like above but with screenshots
* * * stop reading here * * *

only one extra step is copying files/usr into loop file before mounting

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• create a file in MicroSD Card

  ls -la /data/data/com.termux/files
  su
  mkdir /storage/26B7-11F7/com.termux
  cd /storage/26B7-11F7/com.termux
  busybox dd if=/dev/zero bs=1M count=4095 of=usr.img
  

  

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• create a file system (confirm with y)

  /system/bin/mke2fs -t ext4 usr.img
  

  

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• copy files/usr into loop file before mounting

  mkdir -p tmp/usr
  busybox mount usr.img tmp/usr
  
  df -h | grep /dev/loop
  grep termux /proc/mounts
  
  busybox cp -acv /data/data/com.termux/files/usr tmp
  df -h | grep /dev/loop
  

  

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• loop mount the file to existing directory

  umount tmp/usr
  busybox mount usr.img /data/data/com.termux/files/usr
  df -h | grep /dev/loop
  exit
  ls -la /data/data/com.termux/files/usr
  

  

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the same can be repeated for other directories. you can loop mount another home.img file into files/home to extend termux storage

reason for splitting into multiple files is fat32 file size limitation < 4 GB