Friday, December 02, 2005

Default cluster size for FAT and NTFS
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Article ID : 140365
Last Review : November 15, 2005
Revision : 4.0
This article was previously published under Q140365
All file systems used by Windows organize your hard disk based upon cluster (or allocation unit) size, which represents the smallest amount of disk space which can be allocated to hold a file. So when file sizes do not come out to an even multiple of the cluster size, extra space must be used to hold the file (up to the next multiple of the cluster size). On the typical partition, this means that (cluster size)/2 * (number of files) worth of space is lost this way.

If no cluster size is specified during format, NTFS picks defaults based upon the size of the partition. These defaults have been selected to reduce the amount of space lost and to reduce the amount of fragmentation on the partition.
The following default values are used by Windows NT 3.51 and later when a volume is formatted to NTFS via one of the following methods:
1. Using the FORMAT command from the command line without specifying a cluster size.
2. Formatting a volume from Windows Explorer when Allocation Unit box in the format dialog lists "Default Allocation Size".
Cluster size Maximum NTFS volume size (bytes RAW)
512 2,199,023,255,552 (2TB)
1024 4,398,046,511,104 (4TB)
2048 8,796,093,022,208 (8TB)
4096 17,592,186,044,416 (17TB)
The maximum default cluster size under Windows NT 3.51 and later is 4K due to the fact that NTFS file compression is not possible on drives with a larger allocation size. So format will never use larger than 4k clusters unless the user specifically overrides the defaults by using the /A: switch (for command line format) or by specifying a larger cluster size in the format dialog in Windows Explorer.

As versions of Windows NT earlier than 3.51 do not support NTFS file compression, the default cluster sizes will go above 4k. For Windows NT 3.50 and earlier, the following addition to the above table applies:
Cluster size Maximum NTFS Volume size (bytes RAW)
8192 35,184,372,088,832 (35TB)
16384 70,368,744,177,664 (70TB)
32768 140,737,488,355,328 (140TB)
65536 281,474,976,710,656 (281TB)
When you are using the Convert.exe utility to convert to NTFS, Windows always uses a 512-byte cluster size. This is due to the fact that the FAT structures are aligned on 512 byte boundaries, so any larger cluster size would not allow the conversion to function. Note also when formatting a partition under Windows Setup, the partition is first formatted to FAT and then converted to NTFS, so the cluster size will also always be 512 bytes when a partition is formated in Setup.

Note This only applies to Windows NT 3.5, 3.51, and 4.0. It does not apply to Windows 2000 Setup, which formats the partition using the file system you choose.

The FAT file system uses the following cluster sizes. These sizes the same under Microsoft Windows NT, Microsoft MS-DOS, Microsoft Windows 95 and any other operating system that supports FAT:

Drive Size FAT Type Sectors Cluster
(logical volume) Per Cluster Size
----------------- -------- ----------- -------
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 - 2048 MB 16-bit 64 32K
2048 MB - 4096 MB 16-bit 128 64K
*4096 MB - 8192 MB 16-bit 256 128K NT V4.0 only
*8192 MB - 16384 MB 16-bit 512 256K NT V4.0 only

To support > 4GB FAT partitions using 128k or 256k clusters, the drives must use > 512 byte sectors.

Note On very small FAT partitions, a 12-bit FAT is used instead of a 16-bit FAT. The FAT files system only supports 512 byte sectors, so both the sectors per cluster and the cluster size is fixed.

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