Windows® Internals, Sixth Edition, Part 2

Clusters are the addressable blocks that many file system formats use. Cluster size is always a multiple of the sector size, as shown in Figure 12-1. File system formats use clusters to manage disk space more efficiently; a cluster size that is larger than the sector size divides a disk into more manageable blocks. The potential trade-off of a larger cluster size is wasted disk space, or internal fragmentation, that results when file sizes aren’t exact multiples of the cluster size.

Figure 12-1. Sectors and a cluster on a disk

Metadata is data stored on a volume in support of file system format management. It isn’t typically made accessible to applications. Metadata includes the data that defines the placement of files and directories on a volume, for example.

Windows File System Formats

Windows includes support for the following file system formats:

CDFS

UDF

FAT12, FAT16, and FAT32

exFAT

NTFS

Each of these formats is best suited for certain environments, as you’ll see in the following sections.

CDFS

CDFS (%SystemRoot%\System32\Drivers\Cdfs.sys), or CD-ROM file system, is a read-only file system driver that supports a superset of the ISO-9660 format as well as a superset of the Joliet disk format. While the ISO-9660 format is relatively simple and has limitations such as ASCII uppercase names with a maximum length of 32 characters, Joliet is more flexible and supports Unicode names of arbitrary length. If structures for both formats are present on a disk (to offer maximum compatibility), CDFS uses the Joliet format. CDFS has a couple of restrictions:

A maximum file size of 4 GB

A maximum of 65,535 directories

CDFS is considered a legacy format because the industry has adopted the Universal Disk Format (UDF) as the standard for optical media.

UDF

The Windows UDF file system implementation is OSTA (Optical Storage Technology Association) UDF-compliant. (UDF is a subset of the ISO-13346 format with extensions for formats such as CD-R and DVD-R/RW.) OSTA defined UDF in 1995 as a format to replace the ISO-9660 format for magneto-optical storage media, mainly DVD-ROM. UDF is included in the DVD specification and is more flexible than CDFS. The UDF file system format has the following traits:

Directory and file names can be 254 ASCII or 127 Unicode characters long.

Files can be sparse. (Sparse files are defined later in this chapter.)

File sizes are specified with 64 bits.

Support for access control lists (ACLs).

Support for alternate data streams.

The UDF driver supports UDF versions up to 2.60. The UDF format was designed with rewritable media in mind. The Windows UDF driver (%SystemRoot%\System32\Drivers\Udfs.sys) provides read-write support for Blu-ray, DVD-RAM, CD-R/RW, and DVD+-R/RW drives when using UDF 2.50 and read-only support when using UDF 2.60. However, Windows does not implement support for certain UDF features such as named streams and access control lists.

FAT12, FAT16, and FAT32

Windows supports the FAT file system primarily for compatibility with other operating systems in multiboot systems, and as a format for flash drives or memory cards. The Windows FAT file system driver is implemented in %SystemRoot%\System32\Drivers\Fastfat.sys.

The name of each FAT format includes a number that indicates the number of bits that the particular format uses to identify clusters on a disk. FAT12’s 12-bit cluster identifier limits a partition to storing a maximum of 2¹² (4,096) clusters. Windows permits cluster sizes from 512 bytes to 8 KB, which limits a FAT12 volume size to 32 MB.

Note

All FAT file system types reserve the first two clusters and the last 16 clusters of a volume, so the number of usable clusters for a FAT12 volume, for instance, is slightly less than 4,096.

FAT16, with a 16-bit cluster identifier, can address 2¹⁶ (65,536) clusters. On Windows, FAT16 cluster sizes range from 512 bytes (the sector size) to 64 KB (on disks with a 512-byte sector size), which limits FAT16 volume sizes to 4 GB. Disks with a sector size of 4,096 bytes allow for clusters of 256 KB. The cluster size Windows uses depends on the size of a volume. The various sizes are listed in Table 12-1. If you format a volume that is less than 16 MB as FAT by using the format command or the Disk Management snap-in, Windows uses the FAT12 format instead of FAT16.

Table 12-1. Default FAT16 Cluster Sizes in Windows

Volume Size

Default Cluster Size

<8 MB

Not supported

8 MB–32 MB

512 bytes

32 MB–64 MB

1 KB

64 MB–128 MB

2 KB

128 MB–256 MB

4 KB

256 MB–512 MB

8 KB

512 MB–1,024 MB

16 KB

1 GB–2 GB

32 KB

2 GB–4 GB

64 KB

>16 GB

Not supported