Re: Exfat
Velasco Thibault
exFAT (Extensible File Allocation Table) is a file system introduced by Microsoft in 2006 and optimized for flash memory such as USB flash drives and SD cards.[6] exFAT was proprietary until 28 August 2019, when Microsoft published its specification.[7] Microsoft owns patents on several elements of its design.[2]
exFAT was introduced in late 2006 as part of Windows CE 6.0, an embedded Windows operating system. Support was added to regular Windows with Windows Vista Service Pack 1 and Windows Server 2008, both released on February 4, 2008. An update for Windows XP and Windows Server 2003 was later released.
Most of the vendors signing on for licenses are manufacturers of embedded systems or device manufacturers that produce media formatted 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 easily implemented in firmware.
Windows XP requires update KB955704 to be installed, and Windows Vista requires its SP1 installed.[14][15][16] Windows Vista is unable to use exFAT drives for ReadyBoost. Windows 7 removes this limitation, enabling ReadyBoost caches larger than 4 GB.[17] Windows 10 only allows formatting exFAT and NTFS on non-removeable volumes sized larger than 32 GB with the default user interface, and FAT32 format is suggested for smaller volumes; command-line utilities don't accept quick format using FAT32 if volume is larger than 32 GB.
The standard exFAT implementation is not journaled and only uses a single file allocation table and free-space map. FAT file systems instead used alternating tables, as this allowed recovery of the file system if the media was ejected during a write (which occurs frequently in practice with removable media). The optional TexFAT component adds support for additional backup tables and maps, but may not be supported.
The exFAT format allows individual files larger than 4 GB, facilitating long continuous recording of HD video, which can exceed the 4 GB limit in less than an hour. Current digital cameras using FAT32 will break the video files into multiple segments of approximately 2 or 4 GB.
SDXC cards running at UHS-I have a minimum guaranteed write speed of 10 MB/s, and exFAT plays a factor in achieving this throughput through the reduction of the file-system overhead in cluster allocation. This is achieved through the introduction of a separate cluster bitmap where the reservation state of each cluster (reserved/free) is tracked by only one bit, reducing writes to the much larger FAT that originally served this purpose.
Additionally, a single bit in the directory record indicates that the file is contiguous (unfragmented), telling the exFAT driver to ignore the FAT. 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 supported in Windows XP and Windows Server 2003 with update KB955704,[1] Windows Vista Service Pack 1 and later,[19] Windows Server 2008 and later (except Server Core),[20] macOS starting from 10.6.5, Linux via FUSE or natively starting from kernel 5.4, and iPadOS as well as iOS starting from 13.1.[21][22][23]
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.[6]
exFAT is the official file system of SDXC cards. Because of this, any device not supporting exFAT, such as the Nintendo 3DS, may not legally advertise itself as SDXC compatible, despite supporting SDXC cards as mass storage devices by formatting the card with FAT32 or a proprietary file system tied to the device in question.
Some vendors of other flash media, including USB pen drives, compact flash (CF) and solid-state drives (SSD) ship some of their high-capacity media pre-formatted with the exFAT file system to ensure compatibility with any given device. For example, Sandisk ships their 256 GB CF cards as exFAT.
Linux has support for exFAT via FUSE since 2009.[4] In 2013, Samsung Electronics published a Linux driver for exFAT under GPL.[31]On 28 August 2019, Microsoft published the exFAT specification[7] and released the patent to the Open Invention Network members.[32] The Linux kernel introduced native exFAT support with the 5.4 release in November 2019.[33]
exFAT employs a filename hash-based lookup phase to speed certain cases, which is described in US patent Quick File Name Lookup Using Name Hash.[2] Appendix A of the document contains details helpful in understanding the file system.
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 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, which was patented December 10, 2013.[35] 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 on November 3, 2009.[36]
exFAT and the rest of the FAT family of file systems do not use indexes for file names, 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 file names shorter than 16 characters in length, one file name 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 MB 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 minimal three-record file set.) To help improve the sequential searching of the directories (including the root) a hash value of the file name 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 file names are compared to ensure that the proper file was located in case of hash collisions. This improves performance because only 2 bytes have to be compared for each file. This significantly reduces the CPU load because most file names are more than 2 characters (bytes) in size and virtually every comparison is performed on only 2 bytes at a time until the intended file is located.
When the file system is mounted, and the integrity check is conducted, these hashes are verified. Mounting also includes comparison of the version of the exFAT file system by the driver to make sure the driver is compatible with the file system 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 file system can't run if they are missing). If any of these checks fail, the file system should not be mounted, although in certain cases it may mount read-only.
The first feature requires support from the formatting software. Compliant implementations will follow existing offsets. The OEM parameter may be ignored. Implementations may also use TRIM to reduce wear.
exFAT was a proprietary file system until 2019, when Microsoft released the specification and allowed OIN members to use their patents.[43] This lack of documentation along with the threat of a patent infringement lawsuit, as happened previously when Microsoft sued various companies over the VFAT long file name patent (before it expired), hampered the development of free and open-source drivers for exFAT, and led to a situation where Linux distributions could not even tell users how to get an exFAT driver. Accordingly, exFAT official support was effectively limited to Microsoft's own products and those of Microsoft's licensees. This, in turn, inhibited exFAT's adoption as a universal exchange format, as it was safer and easier for vendors to rely on FAT32 than it was to pay Microsoft or risk being sued.
Regardless of whether open-source or not, Microsoft stated that "a license is required in order to implement exFAT and use it in a product or device".[44] Unlicensed distribution of an exFAT driver would make the distributor liable for financial damages if the driver is found to have violated Microsoft's patents.[47][48] While the patents may not be enforceable, this can only be determined through a legal process, which is expensive and time-consuming. It may also be possible to achieve the intended results without infringing Microsoft's patents.[49] In October 2018, Microsoft released 60,000 patents to the Open Invention Network members for Linux systems, but exFAT patents were not initially included at the time. There was, however, discussion within Microsoft over whether Microsoft should allow exFAT in Linux devices,[50][51] which eventually resulted in Microsoft publishing the official specification for open usage[7] and releasing the exFAT patents to the OIN in August 2019.[32]
A FUSE-based implementation named fuse-exfat, or exfat-fuse,[4] with read/write support is available for FreeBSD, multiple Linux distributions, and older versions of Mac OS X. It supports TRIM.[52][53][54][55][56] An implementation called exFATFileSystem, based on fuse-exfat, is available for AmigaOS 4.1.[57]
A Linux kernel implementation by Samsung Electronics is available.[58] It was initially released on GitHub unintentionally,[59] and later released officially by Samsung in compliance with the GPLv2 in 2013.[60][61] (This release does not make exFAT royalty-free, as licensing from Samsung does not remove Microsoft's patent rights.)[62][63] A version of this driver was first incorporated into version 5.4 of the Linux kernel.[64][65][66] A much newer version of the driver, with several bug fixes and improved reliability, was incorporated into kernel 5.7.[67][40] Prior to its being merged into the Linux kernel, this newer version had already seen adoption on Android smartphones and continued to be used on both Linux and Android thereafter.[68][69]
d3342ee215