ScanDisk Recommends Running DEFRAG on a Compressed Drive (109846)

SYMPTOMS

You may receive the following message while ScanDisk is examining your compressed drive

DoubleSpace drive <X> needs to be defragmented. To defragment drive <X>, run the Microsoft Defragmenter on it by typing DEFRAG <X>: at the command prompt.

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 prompt.

Running DEFRAG <X>: (where X is the compressed drive) may not correct the internal fragmentation in all cases.

CAUSE

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.

RESOLUTION

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 drive):

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

MORE INFORMATION

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 drive.

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 contiguous.