Microsoft KB Archive/67321

= FAT Type and Cluster Size Depends on Logical Drive Size =

Article ID: 67321

Article Last Modified on 11/16/2006

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APPLIES TO


 * Microsoft MS-DOS 3.1
 * Microsoft MS-DOS 3.2 Standard Edition
 * Microsoft MS-DOS 3.21 Standard Edition
 * Microsoft MS-DOS 3.3 Standard Edition
 * Microsoft MS-DOS 3.3a
 * Microsoft MS-DOS 4.0 Standard Edition
 * Microsoft MS-DOS 4.01 Standard Edition
 * Microsoft MS-DOS 5.0 Standard Edition
 * Microsoft MS-DOS 5.0a
 * Microsoft MS-DOS 6.0 Standard Edition
 * Microsoft MS-DOS 6.2 Standard Edition
 * Microsoft MS-DOS 6.21 Standard Edition
 * Microsoft MS-DOS 6.22 Standard Edition
 * Microsoft Windows 95

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This article was previously published under Q67321



SUMMARY
MS-DOS allocates disk space for files in units of one or more sectors; these units are called &quot;clusters&quot; or &quot;allocation units.&quot; On any MS-DOS disk, a 1-byte file is allocated 1 cluster of disk space, wasting the unused area of the cluster. A file that is 3.2 clusters large is given 4 clusters. Overall, a smaller cluster size means less waste.

The cluster size for a drive is decided by FORMAT, depending on the size of the logical drive (see table, below). &quot;Logical drive&quot; refers to an MS-DOS volume accessed by a drive letter (A:, B:, C:, D:, and so forth). Hard disk users may want to consider cluster size when choosing how to partition their drive(s).

The cluster size of a floppy drive cannot be changed. The cluster size of a hard drive can be changed only by changing the size of the logical drive, which is done by repartitioning the hard drive.

CHKDSK displays the allocation unit size for a logical drive. FDISK's option 4 displays the size(s) of logical drives on the hard drive(s).

NOTE: DoubleSpace-compressed drives appear to have 8K clusters, but internally vary the sectors-per-cluster as necessary (this information is recorded in the MDFAT). For example, a 10K file which compresses by a factor of 2:1 actually uses 5K, or 10 sectors, of drive space.



MORE INFORMATION
The following is a table of logical drive sizes, FAT (File Allocation Table) types, and cluster sizes:                  Drive Size      FAT Type     Sectors     Cluster (logical volume)             Per Cluster     Size ---  --- (Floppy Disks)      360K           12-bit         2         1K 720K          12-bit         2         1K 1.2 MB         12-bit         1       512 bytes 1.44 MB        12-bit         1       512 bytes 2.88 MB        12-bit         2         1K (Hard Disks)    0 MB - 15 MB      12-bit         8         4K 16 MB - 127 MB    16-bit         4         2K 128 MB - 255 MB    16-bit         8         4K 256 MB - 511 MB    16-bit        16         8K 512 MB - 1023 MB   16-bit        32        16K 1024 MB - 2047 MB   16-bit        64        32K NOTES:
 * Sectors are 512 bytes in size, except on some RAM drives.
 * In the past, some OEMs have modified their versions of MS-DOS to support other sector and/or cluster sizes. The Microsoft MS-DOS 5 Upgrade Setup will, if possible, convert the logical drive to MS-DOS 5.0 compatible. This entails converting the sector size to 512 bytes while retaining the nonstandard cluster size.
 * MS-DOS determines the FAT size based on the number of clusters. If there are 4086 or fewer clusters, a 12-bit FAT is used. If there are 4087 or more clusters, a 16-bit FAT is used.

For more information on this topic, query on the following words:

cluster and disk and FAT

Additional query words: dblspace 3.20 3.21 3.30 3.30a 4.00 4.01 5.00 5.00a partition allocation unit 6.00 6.20 cluster granularity 6.21 6.22 win95 msdos

Keywords: KB67321

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