ExFAT

Overview

exFAT was first introduced in late 2006 as part of Windows CE 6.0, an embedded Windows operating system. Most of the vendors signing on for licenses of exFAT are either for embedded systems or device manufacturers that produce media that will be preformatted with exFAT. The entire File Allocation Table (FAT) family, exFAT included, is used for embedded systems because it is lightweight and is better suited for solutions that have low memory and low power requirements, and can be implemented in firmware.

exFAT allows individual files larger than 4 GiB, facilitating long continuous recording of HD video which can exceed the 4 GiB limit in less than an hour. Current digital cameras using FAT32 will break the video files into multiple segments of approximately 2 or 4 GiB. With the increase of capacity and the increase of data being transferred, the write operation needs to be made more efficient. SDXC cards, running at UHS-I have a minimum guaranteed write speed of 10 MBps and exFAT plays a factor in achieving that throughput through the reduction of the file system overhead in cluster allocation. This is achieved through the introduction of a cluster bitmap and elimination (or reduction) of writes to the FAT table. A single bit in the directory record indicates that the file is contiguous, telling the exFAT driver to ignore the FAT table. This optimization is analogous to an extent in other file systems, except that it only applies to whole files, as opposed to contiguous parts of files.

exFAT is also supported in a number of media devices such as modern flat panel TVs, media centers, and portable media players.

Some vendors of flash media, including USB pen drives, compact flash (CF) and solid state drives (SSD) are shipping from the factory with some of their high capacity media pre-formatted with the exFAT file system. For example, Sandisk ships their 256 GB CF cards as exFAT.

File name lookup

Like NTFS and HFS+, exFAT is a proprietary file system. Microsoft asserts that exFAT is covered by US Patent 8583708, awarded on November 27, 2013, and US Patent 8321439, Quick File Name Lookup Using Name Hash, which is an algorithm used in exFAT to speed up file searches. Microsoft had also applied for, and so far in some cases, received additional US patents on separate components that are used within exFAT. Since Microsoft has not officially released the entire exFAT specification, the Name Hash patent application is a key document in providing internal details in the understanding of the file system, since those details were revealed in Appendix A of the application.

File and cluster pre-allocation

Like NTFS, exFAT can pre-allocate disk space for a file by just marking arbitrary space on disk as 'allocated'. For each file, exFAT uses two separate 64-bit length fields in the directory: the Valid Data Length (VDL) which indicates the real size of the file, and the physical data length.

To provide improvement in the allocation of cluster storage for a new file, Microsoft incorporated a method to pre-allocate contiguous clusters and bypass the use of updating the FAT table and on December 10, 2013 the US patent office granted patent US8606830. One feature of exFAT (used in the exFAT implementation within embedded systems) provides atomic transactions for the multiple steps of updating the file system metadata. The feature, called Transaction Safe FAT, or TexFAT, was granted a patent by the US patent office under US7613738 on November 3, 2009.

Directory file set

exFAT and the rest of the FAT family of filesystems does not use indexes for filenames, unlike NTFS which uses B-trees for file searching. When a file is accessed, the directory must be sequentially searched until a match is found. For filenames shorter than 16 characters in length, one filename record is required but the entire file is represented by three 32-byte directory records. This is called a directory file set, and a 256 MiB sub-directory can hold up to 2,796,202 file sets. (If files have longer names, this number will decrease but this is the maximum based on the minimum 3 record file set.) To help improve the sequential searching of the directories (including the root) a hash value of the filename is derived for each file and stored in the directory record. When searching for a file, the file name is first converted to upper case using the upcase table (file names are case insensitive) and then hashed using a proprietary patented algorithm into a 16-bit (2 byte) hash value. Each record in the directory is searched by comparing the hash value. When a match is found, the filenames are compared to ensure that the proper file was located in case of collisions. This improves performance because only 2 bytes have to be compared for each file. This significantly reduces the CPU cycles because most filenames are more than 2 characters (bytes) in size and each comparison is virtually only performed on 2 bytes at a time until the intended file is located.

Metadata and checksums

exFAT introduces metadata integrity through the use of checksums. There are three checksums currently in use. The Volume Boot Record (VBR) is a 12 sector region that contains the boot records, BIOS Parameter Block (BPB), OEM parameters and the checksum sector. (There are two VBR type regions, the main VBR and the backup VBR.) The checksum sector is a checksum of the previous 11 sectors, with the exception of three bytes in the boot sector (Flags and percent used). This provides integrity of the VBR by determining if the VBR was modified. The most common cause could be a boot sector virus, but this would also catch any other corruption to the VBR. A second checksum is used for the upcase table. This is a static table and should never change. Any corruption in the table could prevent files from being located because this table is used to convert the filenames to upper case when searching to locate a file. The third checksum is in the directory file sets. Multiple directory records are used to define a single file and this is called a file set. This file set has metadata including the file name, time stamps, attributes, address of first cluster location of the data, file lengths, and the file name. A checksum is taken over the entire file set and a mismatch would occur if the directory file set was accidentally or maliciously changed. When the filesystem is mounted, and the integrity check is conducted, these hashes are verified. Mounting also includes comparison of the version of the exFAT filesystem by the driver to make sure the driver is compatible with the filesystem it is trying to mount, and to make sure that none of the required directory records are missing (for example, the directory record for the upcase table and Allocation Bitmap are required and the filesystem can't run if they are missing). If any of these checks fail, the filesystem should not be mounted, although in certain cases it may mount read-only.

The file system provides extensibility through template based metadata definitions using generic layouts and generic patterns.

Restrictive licensing and software patents

Microsoft has not officially released the complete exFAT file system specification and a restrictive license from Microsoft is required in order to make and distribute exFAT implementations. Microsoft also asserts software patents on exFAT which make it difficult to re-implement its functionality in a compatible way without violating a large percentage of them. This renders the implementation, distribution, and use of exFAT as a part of free or open-source operating systems or of commercial software, for which the vendors could not obtain a license from Microsoft, legally difficult, especially in countries that recognize software patents. Although exFAT is now widely supported, initially Mac OS X and many consumer devices could only handle FAT12, FAT16, and FAT32. The situation had formerly rendered exFAT, and flash memory formatted with it, impractical as a universal exchange format. Native Linux support for exFAT is still limited. As of 2010, a working implementation under FUSE exists, which reached version 1.0 in 2013.

Features

The specifications, features, and requirements of the exFAT file system include these:

Adoption

exFAT is supported in Windows XP and Windows Server 2003 with update KB955704, Windows Embedded CE 6.0, Windows Vista with Service Pack 1, Windows Server 2008, Windows 7, Windows 8, Windows Server 2008 R2 (except Windows Server 2008 Server Core), Windows 10, and Mac OS X starting from 10.6.5.

Companies can integrate exFAT into a specific group of consumer devices, including cameras, camcorders, and digital photo frames for a flat fee. Mobile phones, PCs, and networks have a different volume pricing model.

Microsoft has entered into licensing agreements with BlackBerry, Panasonic, Sanyo, Sony, Canon, Aspen Avionics, Audiovox, Continental, Harman, LG Automotive and BMW.

A FUSE-based implementation named fuse-exfat, or exfat-fuse, with read/write support is available for FreeBSD and multiple Linux distributions. A kernel implementation has also been released, written by Samsung. It was initially released on GitHub unintentionally, and later released officially by Samsung in compliance with the GPL. None of the solutions can become an official part of Linux due to the patent encumbered status of the exFAT filesystem. An implementation called exFATFileSystem, based on fuse-exfat, is available for AmigaOS 4.1.

Proprietary read/write solutions licensed and derived from the Microsoft exFAT implementation are available for Android, Linux, and other operating systems from Paragon Software Group and Tuxera.

XCFiles (from Datalight) is a proprietary, full-featured implementation, intended to be portable to 32-bit systems. Rtfs (from EBS Embedded Software) is a full-featured implementation for embedded devices.

Two experimental, unofficial solutions are available for DOS. The loadable USBEXFAT driver requires Panasonic's USB stack for DOS and only works with USB storage devices; the open-source EXFAT executable is an exFAT filesystem reader, and requires the HX DOS extender to work. There are no native exFAT real-mode DOS drivers, which would allow usage of, or booting from exFAT volumes.

Mac OS X Snow Leopard 10.6.5 and later can create, read, write, verify, and repair exFAT file systems.