What file system does the format option in settings > storage use?

Question

I was wondering what file system Android will use when I format my (external) SD Card. Can someone enlighten me?
Is it the one already used or exFAT or FAT32 or manufacturer dependent?

Answer 1

It will format the external micro SD card to FAT32.

Answer 2

TL;DR: Every Android uses FAT32. The Cluster size depends on the release. Existing partitions will be preserved. It will always use quick format.

Long Answer: After some more research here is the process as it was coded in the Android source code:

1. The Setting App displays a few confirmations then uses the Intent: Intent(ExternalStorageFormatter.FORMAT_ONLY)

   in 4.4 KitKat: https://android.googlesource.com/platform/packages/apps/Settings/+/kitkat-release/src/com/android/settings/MediaFormat.java
   in 5 Lollipop: https://android.googlesource.com/platform/packages/apps/Settings/+/lollipop-release/src/com/android/settings/MediaFormat.java
   in 6 they changed this. I will have to look more into it.

2. The ExternalStorageFormatter then uses IMountService to call formatVolume(extStoragePath)

   in 4.4 KitKat: https://android.googlesource.com/platform/frameworks/base.git/+/kitkat-release/core/java/com/android/internal/os/storage/ExternalStorageFormatter.java
   in 5 Lollipop: https://android.googlesource.com/platform/frameworks/base.git/+/lollipop-release/core/java/com/android/internal/os/storage/ExternalStorageFormatter.java

3. This function calls
   mConnector = new NativeDaemonConnector(this, "vold", MAX_CONTAINERS * 2, VOLD_TAG, 25); mConnector.execute("volume", "format", path);

   in 4.4 KitKat: https://android.googlesource.com/platform/frameworks/base/+/kitkat-release/services/java/com/android/server/MountService.java
   in 5 Lollipop: https://android.googlesource.com/platform/frameworks/base/+/lollipop-release/services/core/java/com/android/server/MountService.java
   more info about vold: https://source.android.com/devices/storage/config.html

4. NativeDaemonConnector finally calls the native function to format the partition: Volume::formatVol() This call is without a Volume Label and does not wipe the partition.

   see Volume.cpp, VolumeManager.cpp, CommandListener.cpp
   in 4.4 KitKat: https://android.googlesource.com/platform/system/vold/+/kitkat-release in 5 Lollipop: https://android.googlesource.com/platform/system/vold/+/lollipop-release

5. formatVol decides wether to format the partition (if one already exists) or the whole device (see int Volume::formatVol(bool wipe)). When the latter is the case a new MBR is written. Then Fat::format(devicePath, 0, wipe) is called. (Interesting sidenote: when mounting a volume Android is deleting autorun.inf if it exists: see Volume::protectFromAutorunStupidity())

6. in Fat::format (in Marshmallow it is named Vfat) the actual formatting is done with: /system/bin/newfs_msdos. The parameters are different depending on the Android release, number of sectors is always omitted, wipe is always false:
   • Lollipop and Marshmallow: -F 32 -O android -c 64 -A
   • Kitkat: -F 32 -O android -c 8
   • other systems might differ, see: https://android.googlesource.com/platform/system/vold/ /Fat.cpp or Vfat.cpp

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Appendix: /system/bin/newfs_msdos syntax from https://android.googlesource.com/platform/system/core/+/lollipop-release/toolbox/newfs_msdos.c :

usage: newfs_msdos [ -options ] special [disktype]
where the options are:
    -@ create file system at specified offset
    -A Attempt to cluster align root directory
    -B get bootstrap from file
    -C create image file with specified size
    -F FAT type (12, 16, or 32)
    -I volume ID
    -L volume label
    -N don't create file system: just print out parameters
    -O OEM string
    -S bytes/sector
    -a sectors/FAT
    -b block size
    -c sectors/cluster
    -e root directory entries
    -f standard format
    -h drive heads
    -i file system info sector
    -k backup boot sector
    -m media descriptor
    -n number of FATs
    -o hidden sectors
    -r reserved sectors
    -s file system size (sectors)
    -u sectors/track