FAT

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The File Allocation Table (FAT) filesystem was originally created for use with MS-DOS (and later pre-NT Microsoft Windows); currently a later revision of FAT (FAT32) is used for USB flash disks.^[1] It has made its way over to Linux systems and has official support in the Linux kernel.

As of August 26th, 2019 Microsoft has published the exFAT filesystem specification^[2] which means support for exFAT can be worked into the mainline Linux kernel. Kernel 5.4 includes initial staging code^[3] for exFAT support.

Installation

Kernel

KERNEL Enable FAT support (CONFIG_VFAT_FS)

File systems  --->
   DOS/FAT/NT Filesystems  --->
      < > MSDOS fs support
      <*> VFAT (Windows-95) fs support
      (437) Default codepage for FAT
      (iso8859-1) Default iocharset for FAT
      [ ]   Enable FAT UTF-8 option by default
   -*- Native language support  --->
      (iso8859-1) Default NLS Option
      <*>   Codepage 437 (United States, Canada)
      <*>   NLS ISO 8859-1  (Latin 1; Western European Languages)
      -*-   NLS UTF-8

When planning on mounting FAT partitions, users may need to specify a codepage= option with mount. In the example above the codepage for the United States and Canada is used, however other codepages can be enabled a necessary. Optionally, users can also set a default codepage for FAT in the kernel configuration. Be sure each codepage value which is to be used has been enabled in the kernel.

Note
Using the codepage option via the mount will override the settings used in the kernel.

Avoid setting Default iocharset for fat to UTF-8; it is not recommended. Instead, pass the utf8=true option when mounting FAT partitions (this requires CONFIG_NLS_UTF8 to be enabled in the kernel. For further information see man 8 mount or see the appropriate kernel documentation at /usr/src/linux/Documentation/filesystems/vfat.txt

Emerge

The sys-fs/dosfstools package is needed for FAT userspace utilities:

root #emerge --ask sys-fs/dosfstools

Usage

It should be possible to mount FAT filesystems with the mount command. Resizing could be done using sys-fs/fatresize.

Removal

To schedule removal for the next depclean action:

root #emerge --ask --depclean --verbose sys-fs/dosfstools

Troubleshooting

Slow file transfer speeds

If file transfer speeds are slow (can be viewed using iotop), ensure the filesystem is mounted with the async filesystem option. Edit /etc/fstab (or /etc/autofs/auto.misc when using autofs) system files as needed, likely removing the sync mount option. By default, filesystems are mounted using the async mount option.

Note
The filesystem sync mount option inhibits slower transfer speeds than the default async mount option, and the sync mount option may cause flash media life-cycle shortening! See man mount option sync explanation.

If file transfer speeds are still slow, try remounting the filesystem with the flush mount option:

root #mount -o remount,flush /path/to/mountpoint

Alternative operating system compatible filesystems

Try UDF filesystem using UDFTools, requiring sys-fs/udftools and Linux kernel UDF filesystem driver. Recently code was added to mkudffs for a fix for creating a mock partition increasing compatibility with Microsoft related operating systems. If using an older Linux kernel, ensure block size is set to 512 for increased compatibility. Most options are now default for compatibility, except for the required --bootarea=mbr creating the mock partition.

root #mkudffs --bootarea=mbr --label=your_label /dev/device_file

Try Samsung's F2FS filesystem.

UTF-8/UTF-16 character hardware bugs

Sometimes hardware firmware bugs will occur on embedded devices (eg. car radios) when reading their required formatted FAT/FAT32 filesystems containing UTF-8 characters. A workaround is to ensure initially mounting the FAT filesystem using (current default) mount options codepage=437,iocharset=iso8859-1,shortname=mixed,errors=remount-ro.

For short filenames, codepage=437 is IBM-PC characters or basically ASCII. For long filenames, iocharset=iso8859-1 specifies ASCII. The option shortname=mixed is default, and can also try shortname=win95 option. Of which, are all current defaults. Additionally to further remedy UTF-8/UTF-16 incompatible characters, use a loop with sed to replace all incompatible UTF-8/UTF-16 characters with an underscore or other ASCII character. (See this replace_chars.sh script.)

For reference, this bug was encountered with a Sony car radio. The MEX-GS610BT radio model would hard reset upon attempting to read a USB flash media/drive containing UTF-8/UTF-16 characters.

As mentioned previously, see /usr/src/linux/Documentation/filesystems/vfat.txt, the Linux Kernel vfat source code documentation, for further explanation on whether to use codepage, iocharset, or utf8 mount options.

Unsorted files and folders

When writing files to the FAT/FAT32 filesystem, devices used for reading the filesystem may show the files and folders as unsorted. Commonly, viewer prefer seeing files and folders sorted alphabetically. Install sys-fs/fatsort, and issue the following command:

root #fatsort /dev/device_file

External resources

/usr/src/linux/Documentation/filesystems/vfat.txt - Documentation on the VFAT filesystem included with the Linux kernel sources.
FAT filesystem and Linux - from Wikipedia
FAT - from the Arch Linux wiki

References

[1] ttps://en.wikipedia.org/wiki/File_Allocation_Table

[2] ttps://docs.microsoft.com/en-us/windows/win32/fileio/exfat-specification

[3] ttps://lkml.iu.edu/hypermail/linux/kernel/1909.2/02462.html

[1]

[2]

[3]

FAT

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