April 7, 1993 This information is the complete text of a flier included in an advertisement for a DOS 6.0 book. I thought it might be of interest to anyone responsible for PC support. The book is "The Only DOS Book You'll Ever Need", 2nd Edition, by Doug Lowe, published by Mike Murach & Associates, Inc., 4697 West Jacquelyn Avenue, Fresno, California 93722-6427. ---------------------------------------------------------------------- DoubleSpace or DoubleTrouble? by Doug Lowe When you install DOS 6.0, the Setup program displays a message enticing you to double your disk space simply by typing DBLSPACE at the command line. You can expect that many of your users will accept Microsoft's invitation to the world of on-the-fly disk compression. The support calls will follow soon after. DoubleSpace is a godsend for users who are tight on disk space. But using DoubleSpace is not as simple as Microsoft would have you believe. Many users will be confused about the relationship between the compressed drive and the host drive, particularly with regard to basic operations such as backing up and checking for errors with the CHKDSK command. Should these operations be done against the compressed drive, the host drive, or both? In the case of backup, the answer is to back up the compressed drive, not the host. Thus, in a basic configuration where C is the compressed drive and H is the host, you should back up drive C, not drive H. Because drive H contains the entire contents of the C drive in a hidden file named DBLSPACE.000, you'll be backing up this information twice if you back up both C and H. And if you back up drive H only, your restore options for drive C are limited to restoring the entire drive because you won't be able to selectively restore files to the C drive. As for the CHKDSK command, you should run it against both the compressed drive and the host. Because DoubleSpace maintains a complete directory structure and FAT within the compressed drive, you should periodically verify its integrity by running CHKDSK. And you should also periodically run CHKDSK on the host drive because a FAT or directory error there can render the compressed drive unusable. Another source of confusion is that the DBLSPACE command duplicates functions performed by other DOS commands. For example, the DBLSPACE command has a /CHKDSK switch that seems to perform the same function as the DOS CHKDSK command. Does that mean you should use CHKDSK for uncompressed drives and DBLSPACE /CHKDSK for compressed drives? No. Although the DBLSPACE /CHKDSK command checks the internal structure of a compressed drive, it doesn't check the compressed drive's directory or FAT structure the way the DOS CHKDSK command does. On the other hand, the DOS CHKDSK command checks both the directory and FAT structure and the compressed drive's internal structure. So you can use the DOS CHKDSK command for both compressed and uncompressed drives. DBLSPACE /CHKDSK is unnecessary. The same holds true for the DBLSPACE /DEFRAG option and the new DOS DEFRAG command: the DEFRAG command automatically invokes DBLSPACE /DEFRAG, so you should use it for both compressed and uncompressed drives. DBLSPACE /DEFRAG by itself is unncesssary. Memory Troubles ---------------- Some users will get stung by the memory requirements of DoubleSpace. To support compressed drives, the DoubleSpace file DBLSPACE.BIN must be admitted into internal memory. Unfortunately, you may not be able to afford the 44KB admission price. The good news is that the DOS 6.0 version of EMM386.EXE does a better job of recovering upper memory than previous versions. Enough so that it can probably find enough new upper memory to accomodate DBLSPACE.BIN with room to spare. Then, all you have to do to load DBLSPACE.BIN into upper memory is make sure your CONFIG.SYS file contains this line: devicehigh=c:\dos\dblspace.sys /move Of course, you must also include Device commands to load HIMEM.SYS and EMM386.EXE. If you're using a third-party memory manager like QEMM-386 or 386Max, there might be some bad news. These programs have probably already recovered as much or more upper memory than can be recovered by EMM386.EXE. If you've already filled that upper memory with device drivers or memory-resident programs, you may not have 44KB to spare for DBLSPACE.BIN. In that case, DoubleSpace will encroach upon your conventional memory. And depending on the application programs you use, that may be unacceptable. What's the best way to deal with the memory issues raised by DoubleSpace? I recommend you run the Mem command before installing DoubleSpace. Near the bottom of its display, it shows the amount of upper memory available. If this amount is greater than 44KB, there's nothing to worry about: there's plenty of room for DBLSPACE.BIN. If Mem shows that you have less than 44KB of free upper memory, run MemMaker, the new DOS 6.0 utility that optimizes your memory use. Then, run Mem again to see if you now have 44KB of free upper memory. If you do, you can safely install DoubleSpace. If you still don't have enough upper memory, maybe you can remove a device driver or memory-resident program you don't really need. Short of that, you simply have to decide whether of not you're willing to trade 44KB of conventional memory for doubled disk space. Drive Letter Troubles --------------------- One final problem that DoubleSpace is likely to cause is drive letter conflicts. For example, suppose you install DoubleSpace on a computer that's attached to a Lantastic network in which drive H is redirected to a network drive. When DoubleSpace compresses drive C, it appropriates drive letter H to use for the host drive. This conflicting use of drive H is bound to cause problems. Besides networking, the use of RAM drives, CD-ROM drives, or removable drives can also lead to drive letter conflicts. The easiest way to resolve these conflicts is simply to assign higher drive letters to your network drives, RAM drives, and so on. Bottom Line ----------- The bottom line is that DoubleSpace is going to be more trouble than Microsoft would have you believe. Still, when you consider the benefits of using DoubleSpace over using other compression software or purchasing larger disk drives, DoubleSpace is worth it. Just be aware that it will raise a host of support issues. --------------------------- End of Text --------------------------