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FAT Type and Cluster Size Depends on Logical Drive Size
Article ID: 67321 - View products that this article applies to.
This article was previously published under Q67321
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-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). "Logical drive" 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.
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
cluster and disk and FAT
Article ID: 67321 - Last Review: November 16, 2006 - Revision: 2.1