exFAT

exFAT (Extended File Allocation Table) is a proprietary file system designed especially for flash drives^[3] developed by Microsoft, which hasapplied for patent protection^[1]. It is supported in Windows XP and Windows Server 2003 with update KB955704,^[2] Windows Embedded CE 6.0,Windows Vista with Service Pack 1,^[4] Windows Server 2008,^[5] Windows 7, Windows Server 2008 R2 (except Windows Server 2008 Server Core),Mac OS X Snow Leopard starting from 10.6.5^[6], Mac OS X Lion and OS X Mountain Lion.

exFAT can be used where the NTFS file system is not a feasible solution, due to data structure overhead, or where the file size limit of the standardFAT32 file system (without FAT+ extension^[7]) is unacceptable.

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

Advantage

The advantages over FAT file systems include:

• For the typical user, seamless interoperability between Windows and OS X platforms for files in excess of 4 GiB. NOTE: Windows XP may require Update for Windows XP (KB955704) to manage exFAT formatted drives.
• Scalability to large disk sizes: 64 ZiB^{[nb 2]} theoretical max, 512 TiB recommended max, raised from the 2 TiB (16 TiB with 4 KiB sectors) limit of FAT32 partitions.^[8]
• File size limit of 16 EiB–1 byte.^{[nb 1]} (limited by volume size), raised from 4 GiB−1 byte in a standard FAT32 filesystem.^[2] (The open FAT+^[7]specification proposes how to store files larger than 4 GiB on FAT32 volumes as well, but this extension is only implemented in some versions of DR-DOS and FreeDOS so far.)
• Cluster size up to 32 MiB.^[2]
• Free space allocation and delete performance improved due to introduction of a free space bitmap.
• Timestamp granularity of 10 ms (same as in FAT12/FAT16/FAT32 for creation time, but not as fine as NTFS's 100 ns).^[1]
• Optional support for UTC timestamps (starting with Vista SP2).^[9]
• Optional support for access control lists (not supported in Windows Vista SP1).^[10]
• Optional support for TexFAT, a transactional file system standard (optionally WinCE activated function).
• Provision for OEM-definable parameters to customize the file system for specific device characteristics

Disadvantage

The disadvantages compared to FAT file systems include:

• Microsoft has not released the official exFAT file system specification, and a restrictive license from Microsoft is required in order to make and distribute exFAT implementations. Microsoft also asserts patents on exFAT which make it impossible to re-implement its functionality in a compatible way without violating a large percentage of them.^[11] This renders the implementation, distribution, and use of exFAT as a part offree or open-source operating systems or of commercial software, for which the vendors could not obtain a license from Microsoft, not only technically difficult, but legally impossible in countries that recognize United States software patents.
• Limited or no support outside Windows and Mac OS X operating systems as of 2012—most consumer electronic devices could only handleFAT12/FAT16/FAT32, rendering exFAT (and flash memory formats using it) impractical as a universal exchange format. (This may change ifSDXC cards and Memory Stick XC come into wide use, as they define exFAT as the file system to be used in their specifications even though this is not a technical requirement.)
• Windows XP and Windows Server 2003 users must have Service Pack 2 or later and install an update to support exFAT.
• Windows Vista must be Service Pack 1 or later for exFAT support.
• Drives formatted using exFAT cannot be read by any version of Windows prior to Windows XP or by any version of DOS or OS/2 (unless emulated as otherwise).
• Windows Vista is unable to use exFAT drives for ReadyBoost. Windows 7 removes this limitation, enabling ReadyBoost caches larger than 4 GiB.^[12]
• The standard exFAT implementation 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.
• Support for up to 2,796,202 files per subdirectory only.^{[1][nb 3]} While Microsoft documents a limit of 65,534 files per sub-directory for their FAT32 implementation, technically there is no such limit and FAT32 implementations in other operating systems allow an unlimited number of files up to the number of available clusters (that is, up to ca. 268,304,373 files, 2²⁸ - 11^[13] - 131,072^[14]).
• Reduced file data I/O performance compared to FAT32 due to smaller cluster sizes and more complex data structures.^{[citation needed]}
• More complex design results in higher code footprint and memory usage compared to FAT implementations. FAT12/FAT16/FAT32 can be implemented in as little as a 512 bytes code and 1 KB data for low-performance read-only versions and a few kilobytes code plus a few hundred KB data for high-performance read-write versions, allowing native support for them even in light-weight operating systems such as DOS, thereby enabling their usage also on older PCs and in small embedded systems. exFAT implementations require magnitudes more system resources rendering this approach impractical.

資料來源: http://en.wikipedia.org/wiki/ExFAT