Largest Possible DoubleSpace Drive Is 512 MB (95533)

The information in this article applies to:
    Microsoft MS-DOS operating system 6.0
    Microsoft MS-DOS operating system 6.2
    Microsoft MS-DOS operating system 6.21
    Microsoft MS-DOS operating system 6.22
    Microsoft Windows 95
    Microsoft Plus! for Windows 95

This article was previously published under Q95533


The versions of DoubleSpace and DriveSpace included with MS-DOS and Windows 95 support cluster sizes up to 8K. This limits the size of a DoubleSpace or DriveSpace compressed volume file (CVF) to 512 MB.

Because DriveSpace 3 (the version of DriveSpace included with Microsoft Plus!) can use cluster sizes up to 32K, the maximum size for a DriveSpace 3 CVF is 2 GB.


The following calculations are used to determine the limits stated above:

DoubleSpace and DriveSpace

   # of clusters with FAT16      max cluster size      max CVF size
   ------------------------  *  ------------------  =  ------------
        65536 clusters          8192 bytes/cluster        512 MB

DriveSpace 3

   # of clusters with FAT16      max cluster size       max CVF size
   ------------------------  *  -------------------  =  ------------
        65536 clusters          32768 bytes/cluster        2 GB
DriveSpace 3 achieves a higher compression ratio by compressing and storing data in clusters up to 32K in size, and by offering two new levels of compression called HiPack and UltraPack. For information about the HiPack and UltraPack compression methods, search for "HiPack" and "UltraPack" in DriveSpace 3 online Help.

DriveSpace 3 can also store more data than earlier versions of DriveSpace and DoubleSpace because of improved fragmentation handling. In DoubleSpace and DriveSpace CVFs, the data for a cluster must be stored in contiguous sectors. When the volume is highly fragmented, it is possible to have many free sectors, but not enough contiguous sectors to store a cluster (up to 16 sectors in a DoubleSpace or DriveSpace CVF, and up to 64 sectors in a DriveSpace 3 CVF). DriveSpace 3 stores data for a fragmented cluster in a linked list of sectors when there are not enough continuous sectors to store the entire cluster. In addition, DriveSpace 3 stores small directories in a single sector, while DoubleSpace and DriveSpace require 16 sectors.

All versions of DoubleSpace and DriveSpace use Sector Allocation Granularity to increase data storage capacity. For additional information about this topic, please see the following article in the Microsoft Knowledge Base:

98407 How DoubleSpace Uses Sector Allocation

For information about cluster sizes, please see the following article in the Microsoft Knowledge Base:

67321 FAT Type and Cluster Size Depends on Logical Drive Size

For information about DoubleSpace and DriveSpace in MS-DOS, please see Chapter 5 of the "Microsoft MS-DOS 6 User's Guide," or see Appendix E of the "Microsoft MS-DOS 6.22 User's Guide."

Modification Type: Major Last Reviewed: 12/17/2000
Keywords: KB95533