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FAT Type and Cluster Size Depends on Logical Drive Size

This article was previously published under Q67321
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This article was written about products for which Microsoft no longer offers support. Therefore, this article is offered "as is" and will no longer be updated.
MS-DOS allocates disk space for files in units of one or more sectors;these units are called "clusters" or "allocation units." On any MS-DOSdisk, a 1-byte file is allocated 1 cluster of disk space, wasting theunused area of the cluster. A file that is 3.2 clusters large is given4 clusters. Overall, a smaller cluster size means less waste.

The cluster size for a drive is decided by FORMAT, depending on thesize of the logical drive (see table, below). "Logical drive" refersto an MS-DOS volume accessed by a drive letter (A:, B:, C:, D:, and soforth). Hard disk users may want to consider cluster size whenchoosing how to partition their drive(s).

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

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

NOTE: DoubleSpace-compressed drives appear to have 8K clusters, butinternally vary the sectors-per-cluster as necessary (this informationis recorded in the MDFAT). For example, a 10K file which compressesby a factor of 2:1 actually uses 5K, or 10 sectors, of drive space.
The following is a table of logical drive sizes, FAT (File AllocationTable) 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				
  • 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
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

Article ID: 67321 - Last Review: 11/16/2006 05:38:18 - Revision: 2.1

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