Article ID: 67321
Article Last Modified on 11/16/2006
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
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 "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.
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
