FATX
From Wiki-Scene
The file system used for formatting the Xbox HDD and memory cards, which loosely resembles FAT32.A file system determines how files are stored on a drive. A PC running Windows, for example, can read drives formatted using either FAT, FAT32, or NTFS - to read other file systems (such as FATX) special software must be used. One such example is Xplorer360.
Files stored on a partition using FATX are limited to 42 characters in length (including the file extension, which usually accounts for 4 characters (eg. ".mp3")). They may be no more then 4gb in size, and only 4096 files (two to the power of twelve) may be stored in any one folder.
Furthermore, the following characters are illegal for use in filenames:
=?:;"*+,/|
Various FATX-compatibility renaming tools are available for the PC, though the XBMC dashboard will automatically alter filenames for you if you use that for file transfers.
Cluster Based Corruption
FATX is cluster based, that is to say, files are split into small "chunks" of multiple kilobytes, and the index table tracks their locations (as opposed to the location of each and every byte of information). This leads to a smaller index (and is the way most file systems work), however FATX has a limitation in that it can only handle up to 16,777,216 (two to the power of twenty four) clusters per partition. If you attempt to add data to the drive to the point where more clusters would be required, the index table overflows and starts overwriting previous entries.
By default, a cluster size of 16kb is used. Multiplying things out (and reverting to plain english), this means if you attempt to store more then 256gb worth of data on any one partition, many (if not all) of your files will corrupt or simply disappear completely!
The solution is to use a larger cluster size, meaning that less index entries are required per file (on average). 32kb clusters allow to safe allocation of up to 512gb worth of data, while 64kb clusters (the largest supported) can be used to create a 1tb partition. Given that just two partitions on an Xbox drive can be of a user-allocated size, this means the largest drive that can be used in the system is 2tb total.
XBPartitioner must be used to format partitions larger then 256gb (as it is the only tool with control over cluster sizes), and your BIOS must support it's use of custom partitions in addition to LBA48.
Note that the "blocks" the MS Dash's memory manager refers to are equal in size to a 16kb cluster. As the partition used for saved data is around 4.76gb (and, of course, uses 16kb clusters), this means that an "empty" drive has around 300,000 free "blocks" on it.
Slack Space & Fragmentation
Note that only one file segment can be stored in any given cluster (though it may take multiple clusters to account for a complete file). This means that if you copy a 1kb file to a drive, it'll take up drive space equal to the size of one full cluster (that is, at least 16kb). This wastage is common to all file systems ("slack space"), though the larger your cluster size and the more files are on the drive, the more pronounced it'll be. By contrast, volumes formatted with NTFS seldom use clusters larger then 4kb.
On average, the amount of space wasted on any given drive will be equal to the amount of files held multiplied by the cluster size and divided by two.
Another side effect of FATX (also common to other systems) is file fragmentation - over time, new files added to the drive will have their clusters scattered in order to fit within the ever decreasing gaps of the drive's free space. A heavily fragmented file requires the drive head to work harder in order to read it (think of it like reading a book that has one page in each room of your house), slowing access times. Normally a defragmenting tool could be used to consolidate files ("put the pages back together"), though no such tool has been developed for FATX as yet.
See Also
- Xbox Drive Formatting
- Differences between Xbox FATX and MS-DOS FAT at Xbox-Linux.org - A more detailed look into the technical workings of FATX.
