FAT File Systems

Question 1

File System Sectors:

Answer 1

The smallest allocation unites provided by devices, conventionally 512 bytes. All low-level I/O disk operations occur at least on sectors

Question 2

File System Clusters:

Answer 2

Most file systems group sectors into clusters to make allocation units manageable. Cluster size varies with file systems, often also with actual file system size. Whether entire clusters are overwritten when partially-filled ones are written out is implementation dependent

Question 3

File Allocation Table (FAT) file system was based loosely on the CP/M file system but added…

Answer 3

Directory support and byte-oriented file lengths

Question 4

FAT Key Characteristics:

Answer 4

Use of a table to keep track of which areas of a volume belong to a file, are free, or not usable and locations on the underlying medium (leaving remainder of the last cluster as slack space)

Question 5

To reduce the size of table entries, clusters of sectors are used and the size of each cluster varies with…

Answer 5

The size of the file system and the maximum number of clusters varies with the different versions of FAT

Question 6

FAT32 cluster size ranges:

Answer 6

4KBytes - 32 KBytes

Question 7

The FAT32 FSINFO structure contains hints for more efficient storage management:

Answer 7

• Primary information on the number of free clusters and the next free cluster
• This information is redundant and only used for performance purposes - but the signature strings for FSINFO

Question 8

Entries in the actual FAT define file allocation chains, each link contains an index to the next cluster for a given file…

Answer 8

• The last entry of a chain is filled with a reserved value
• Similarly, bad clusters are marked with special values
• Unused clusters are marked with 0 values
• For FAT12/16, only 512 entries are reserved for root directory entries at the start of the volume
• In FAT32, the root directory size is variable and starts in cluster 2

Question 9

Each directory entry has a fixed size, resulting in some entries occupying multiple records:

Answer 9

• Each directory entry contains file name and meta-data for a file or directory and is located in clusters allocated to the file’s parent if it is not in the root directory
• The basic file name structure only supports 8.3 ASCII characters using conventions dating back to Digital OS/8-11 and also used e.g. by CP/M
• A longer file name or one requiring Unicode encoding results in additional directory entries being created, preceding the main entry

Question 10

Simple allocation/de-allocation strategy of FAT means that available cluster must be looked up by searching the FAT linearly…

Answer 10

• This is a highly resource-intensive task
• exFAT - provides additional data structures to identify contiguous free areas, while Windows NT attempts to allocate larger segments in advance

Question 11

Even first writing on a new file system, clusters are not necessarily written out contiguously…

Answer 11

• Writes by multiple applications and cached data may be interleaved, particularly on multi-tasking systems
• After reclaiming clusters of deleted files, file operations must traverse these fragments

Question 12

Extended File Allocation Table (exFAT) was introduced as a propietary FAT variant maily for use with high-capacity but…

Answer 12

Slower storage systems such as Flash drives or where the overhead of more elaborate file systems is not acceptable

Question 13

exFAT Attributes:

Answer 13

• The allocation is now based on a combination of a conventional linked list with a bitmap to minimise space consumption
• Directory structures still rely on a simple table
• Bad blocks are marked on a per-cluster basis
• File names can be up to 255 characters long, with basic meta-data for dates, but now supporting 10ms UTC timestamps rather than the 2s found in FAT
• The file system also supports access control lists rather than only basic bit-masks
• To minimise fragmentation, exFAT can reserve data by pre-allocating clusters, for this separate fields are used for the real and reserved data length

Question 14

Removal of some restrictions found in FAT32:

Answer 14

• The use of an allocation bit-map improves allocation of contiguous file extends, significantly reducing fragmentation
• FAT32 limits file sizes to 4GB
• Volumes are limited to 32GB by some FAT32 implementations such as WindowsXP, workarounds are inefficient and inelegant
• exFAT supports 64 bit values for files sizes
• To support large numbers of files, simple per-directory hash tables are used to accelerate look-ups
• Unlike FAT, exFAT uses checksums to protect boot records and directory file sets