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@@ -0,0 +1,215 @@ +This file gives answers to frequently asked questions about dmsdos. + + +What is dmsdos ? + +Dmsdos is a Linux kernel module that allows access to msdos/win95 compressed +disk partitions. It supports different doublespace, drivespace and stacker +versions. +See file dmsdos.doc for an actual list of supported dos/win95 configurations. + + + +How do I mount a compressed partition? + +You need to mount the uncompressed host partition as usual. In this +partition, you can see the compressed partiton as large file (the CVF, +Compressed Volume File). It usually has names like 'dblspace.NNN', +'drvspace.NNN' or 'stacvol.XXX'. The whole compressed partition is inside +this file. + +You need the loopback block device driver (enable the loopback device during +kernel configuration): + + * + * Additional Block Devices + * + Loopback device support (CONFIG_BLK_DEV_LOOP) [M/n/y/?] Y or M + +The loopback block device allows mounting a file as a filesystem (read some +Linux docs about this for more information). To avoid confusion, note +that this has absolutely nothing in common with the well-known network +loopback interface. + +Example: 'mount -t msdos -o loop /DOS/dblspace.001 /mnt' + +This mounts your compressed filesystem that in fact resides in the file +/DOS/dblspace.001 under /mnt. + + + +It doesn't work! + +Well, nothing simply doesn't work. It fails at a specific point and gives +some errors or warnings before failure which are usually an important hint +for finding out what exactly went wrong. Please have a look at file +troubleshooting.doc. If you still cannot solve the problem you can send an +email to the current dmsdos maintainer. The email address and a checklist +what to include in a bug report can be found in file dmsdos.doc. + + + +I have compressed partitions, but I'm using umsdos. Can I use dmsdos without +losing umsdos features? + +No problem. It should work out of the box. If you want to use umsdos style +long filenames inside the CVF, you can even mount it as type umsdos instead +of msdos. (Uhh. Use kernel 2.0.xx or get at least 2.1.94 for that). + + + +My compressed partition has long filenames under Win95. How can I see +them with dmsdos ? + +Mount as type vfat instead of msdos. + + + +Can I compress my Linux data under dmsdos? + +You can (via umsdos), but it's not recommended. + +Just to let you know: There's some other compression software available that +may be a better choice for Linux data. See file dmsdos.doc. + + + +Can I boot Linux from a compressed partition? + +No. (Older dmsdos versions supported it, but this feature has been given up +when the CVF-FAT interface was introduced. Well, the documentation always +warned ... ) + + + +Uhhh. Write access is so ssssssslow..... + +What can I say. You decided to use compressed partitions - now you are +experiencing one of their greatest disadvantages. But dmsdos has some +special tricks for you in this case. You can... (in recommended order) + + - run the dmsdos daemon which does delayed compression in the background + when the system is idle, or use the internal daemon + + - set speedup bit #2 (allocation code skips exact search) + + - set speedup bit #6 (allocation code doesn't search at all but takes + any hole free hole) (Really Not Recommended if you don't know what + you are doing). Read the comments in the documentation, please. + +See the dmsdos documentation for details and a discussion about their +advantages and disadvantages. + + + +Is it safe to use dmsdos? + +This question is something like that one: 'is it safe to drive a car?'. + +There's no warrenty. I really can't promise that there aren't any bugs in +the driver code. I think you just like to feel safe: + + - If you don't have to write to your compressed filesystems, mount + read-only. + + - Run the dos filesystem checker every time you boot dos and backup from + time to time. + + - Backup your compressed filesystem before trying to mount read-write. + + + +How do I enable/disable long filename support? + +For long filename support use a filesystem type that knows long filenames, +e.g. vfat instead of msdos. + + + +What happens if the compressed filesystem gets full during write access? + +Don't ask. That has become too complex. :( + +Well, in the worst case the same thing happens as under Dos: you get a +strange error (Dos: "sector not found") and might lose data. It is difficult +because some applications do not check the return values of write access +functions (AAARGHHH!!!). On a compressed filesystem not only cluster +allocation, but also usual write access may fail (i.e. when you replace +data that compress well by data that don't compress well and thus need more +physical space). So just keep in mind that you shouldn't fill a compressed +partition up to the last byte :) + + + +What's the difference between win95 doublespace and drivespace 3 ? + +Please note that win95 has two different drivespace programs. One of them +is included in win95 and it's in fact an old dos doublespace version. I +still refer to it as 'win95 doublespace' though M$ call it 'drivespace'. +This one is supported without restrictions. + +The other one is sold seperately from win95 in a 'M$ Plus' package (is this +still true?) M$ have called it Drivespace 3. Meanwhile it should also +be supported quite well. + +If you aren't sure, watch the filename of the large, hidden CVF. If it is +something like 'dblspace.xxx' it's the old version. If it's 'drvspace.xxx' +instead and you are very sure you created it under win95, it's Drivespace 3. +Dmsdos also tells when it detects a drivespace 3 format CVF. + +The main difference between these two versions is that drivespace 3 uses a +larger cluster size (32KB instead of 8KB) and can compress up to 2GB of data +instead of only 512KB. It has also a more advanced compression scheme +(SQ-0-0). + + + +When do I have to defragment my compressed partitions? + +Watch the dutil output (see file dmsdos.doc for a dutil description). It +displays a fragmentation value. You should keep it below 75% to feel safe. +When dutil even displays a warning about high fragmentation, you should +immediately boot Dos and defragment the CVF. + + + +Does dmsdos work with FAT32 filesystems? + +Yes. All earlier problems have been avoided by moving dmsdos one layer down +in the filesystem interface. Well, it does not support *compressed* FAT32 +filesystems. Win95 doesn't do either :) But, if some day they show up to +exist, I might be persuaded to implement it also. + + + +What about the relationship between dmsdos and software patents ? + +I'm still receiving mails about this question. At my best knowledge dmsdos +does not violate any software patents. (Well, I don't hope so, but my best +knowledge might be wrong. I'm not a lawyer.) If you are interested in +details take a look at file patent.doc. + + + +Do I have to recompile dmsdos when I upgrade my kernel ? + +The safe answer is yes. It's best to recompile *all* modules if you change +something in your kernel though not always necessary. But who knows. + +Some clever people invented a 'module version information' trick. You can +enable it during kernel configuration. This makes your modules less kernel +version dependent e.g. a module compiled under kernel 2.0.33 may also work +under kernel 2.0.34. In that case, you may share one precompiled dmsdos +module with several kernels. But please don't overuse it. You should not, +for example, use the same compiled object code of a dmsdos module for +kernel 2.0.34 and 2.1.105 - this is very likely to cause problems :) + + + +Where does the name "DMSDOS" come from ? + +It has historical reasons. The first piece of dmsdos code was written in +times of MSDos 6.2 when Doublespace became very popular. Thus, dmsdos was +meant as something like "doublespaced msdos". Today, it supports a lot of +more compressed Dos filesystems, and a better name would be "cmsdos" for +"compressed msdos". But you never change the name of a successful project :) |