ScanDisk Recommends Running DEFRAG on a Compressed Drive (109846)
The information in this article applies to:
• Microsoft MS-DOS operating system 6.2
• Microsoft MS-DOS operating system 6.22
This article was previously published under Q109846
This information applies to both Microsoft DoubleSpace and Microsoft
DriveSpace. For MS-DOS 6.22, use DRVSPACE in place of DBLSPACE for commands
You may receive the following message while ScanDisk is examining your
DoubleSpace drive <X> needs to be defragmented. To defragment drive <X>,
run the Microsoft Defragmenter on it by typing DEFRAG <X>: at the
where X is the compressed drive. Choosing the More Information button
displays the following:
DoubleSpace drive <X> is becoming highly fragmented internally. When a
compressed drive becomes fragmented in this way, some free space becomes
unusable until the drive is defragmented. To defragment drive <X>, run
the Microsoft Defragmenter on it by typing DEFRAG <X>: at the command
Running DEFRAG <X>: (where X is the compressed drive) may not correct the
internal fragmentation in all cases.
This message is displayed when 50 percent or more of the free space on the
compressed drive is fragmented. If all of the free space becomes
fragmented, DoubleSpace cannot write data to the compressed drive, even
though free space remains in the sector heap.
To determine the percentage of internal fragmentation on a compressed
drive, type dblspace /info <X>: (where X is the compressed drive) at the
MS-DOS command prompt.
In most instances, running Microsoft Defragmenter (Defrag) on the
compressed drive corrects this problem. Defrag runs DBLSPACE /DEFRAG to
consolidate free space at the end of the compressed volume file (CVF). If
this does not resolve the ScanDisk warning, thoroughly defragment your
compressed drive using the following procedure (where <X> is the compressed
• Run DBLSPACE /DEFRAGMENT <X>:
This moves all free space to the end of the CVF.
• Run DBLSPACE /DEFRAGMENT /F <X>:
This moves all free space to the front of the CVF.
• Run DBLSPACE /DEFRAGMENT <X>:
This moves all free space back to the end of the CVF.
NOTE: This procedure may take several hours to complete, depending on the
size of the drive and level of fragmentation. You may want to place the
above commands in a batch file and let it run overnight.
All the unused sectors should then be contiguous in the sector heap. For
more information on this procedure, query on the following words in the
Microsoft Knowledge Base:
dblspace and defragment and sectors and free
DoubleSpace compresses data one FAT cluster (8 kilobytes[K]) at a time and
must write the data to the sector heap in contiguous sectors. If the data
is not compressible at all, DoubleSpace requires 8K (16 sectors) of
contiguous sectors in the sector heap to write the data. If there are not
16 available contiguous sectors in the sector heap, DoubleSpace cannot
write the data to the compressed drive until the drive is defragmented.
When 50 percent or more of the free space in the CVF cannot be mapped to 8K
of contiguous sectors in the sector heap, ScanDisk displays the above
message. ScanDisk and DBLSPACE /INFO use the same algorithm for reporting
internal CVF fragmentation. If 100 percent of the free space in the sector
heap is fragmented, DoubleSpace may be unable to write any more data to the
Every CVF contains a standard 12-bit or 16-bit MS-DOS file allocation table
(FAT) with a cluster size of 8192 bytes (8K). The cluster size cannot be
changed. DoubleSpace maps each 8K FAT cluster to a Microsoft DoubleSpace
FAT (MDFAT) entry. Each MDFAT entry maps to individual sectors in the
sector heap, and the sectors used for an individual MDFAT entry must be
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