DOCUMENT:Q67321 17-DEC-2000 [msdos] TITLE :FAT Type and Cluster Size Depends on Logical Drive Size PRODUCT :Microsoft Disk Operating System PROD/VER:MS-DOS:3.x,4.x,5.x,6.0,6.2,6.21,6.22; Win2000:95 OPER/SYS: KEYWORDS:msdos ====================================================================== ------------------------------------------------------------------------------- The information in this article applies to: - Microsoft MS-DOS operating system versions 3.1, 3.2, 3.21, 3.3, 3.3a, 4.0, 4.01, 5.0, 5.0a, 6.0, 6.2, 6.21, 6.22 - Microsoft Windows 95 ------------------------------------------------------------------------------- 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 4085 or fewer clusters, a 12-bit FAT is used. If there are 4086 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 : msdos Technology : kbWin95search kbZNotKeyword3 kbMSDOSSearch kbMSDOS321 kbMSDOS400 kbMSDOS320 kbMSDOS330a kbMSDOS621 kbMSDOS622 kbMSDOS620 kbMSDOS600 kbMSDOS310 kbMSDOS500 kbMSDOS330 kbMSDOS401 kbMSDOS500a Version : MS-DOS:3.x,4.x,5.x,6.0,6.2,6.21,6.22; Win2000:95 ============================================================================= THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY. Copyright Microsoft Corporation 2000.