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
- 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